diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index c178671d56..25fcef4914 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -105,7 +105,7 @@ jobs:
         run: |
           docker pull nfcore/sarek:dev
           docker tag nfcore/sarek:dev nfcore/sarek:dev
-      - name: Run targeted and splitfastq tests
+      - name: Run ${{ matrix.profile }} test
         run: |
           nextflow run . -profile ${{ matrix.profile }},docker --verbose
   tools:
@@ -125,6 +125,6 @@ jobs:
         run: |
           docker pull nfcore/sarek:dev
           docker tag nfcore/sarek:dev nfcore/sarek:dev
-      - name: Run variant calling test on specific tools
+      - name: Run ${{ matrix.tool }} test
         run: |
           nextflow run . -profile test_tool,docker --verbose --tools ${{ matrix.tool }}
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3cdb1cc196..ca00e11eed 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,14 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/) a
 
 ## dev
 
+### `Added`
+
+- [#76](https://github.com/nf-core/sarek/pull/76) - Add `GATK Spark` possibilities to Sarek
+
+### `Changed`
+
+- [#76](https://github.com/nf-core/sarek/pull/76) - Use `MarkDuplicatesSpark` instead of `MarkDuplicates`
+- [#76](https://github.com/nf-core/sarek/pull/76) - Use `gatk4-spark` instead of `gatk4` in `environment.yml`
 - [#80](https://github.com/nf-core/sarek/pull/80) - Re-bump `dev` branch
 
 ## [2.5.2] - Jåkkåtjkaskajekna
@@ -25,6 +33,7 @@ Jåkkåtjkaskajekna is one of the two glaciers of the Ålkatj Massif.
 - [#60](https://github.com/nf-core/sarek/pull/60) - Add new minimal genomes (`TAIR10`, `EB2`, `UMD3.1`, `bosTau8`, `WBcel235`, `ce10`, `CanFam3.1`, `canFam3`, `GRCz10`, `danRer10`, `BDGP6`, `dm6`, `EquCab2`, `equCab2`, `EB1`, `Galgal4`, `galGal4`, `Gm01`, `hg38`, `hg19`, `Mmul_1`, `mm10`, `IRGSP-1.0`, `CHIMP2.1.4`, `panTro4`, `Rnor_6.0`, `rn6`, `R64-1-1`, `sacCer3`, `EF2`, `Sbi1`, `Sscrofa10.2`, `susScr3`, `AGPv3`) to `igenomes.config`
 - [#61](https://github.com/nf-core/sarek/pull/61) - Add params `split_fastq`
 - [#61](https://github.com/nf-core/sarek/pull/61) - Add test `SPLITFASTQ`
+- [#66](https://github.com/nf-core/sarek/pull/66) - Add `Sentieon` possibilities to Sarek
 
 ### `Changed`
 
@@ -41,6 +50,7 @@ Jåkkåtjkaskajekna is one of the two glaciers of the Ålkatj Massif.
 
 - [#46](https://github.com/nf-core/sarek/pull/46) - Remove mention of old `build.nf` script which was included in `main.nf`
 - [#74](https://github.com/nf-core/sarek/pull/74) - Remove `download_image.sh` and `run_tests.sh` scripts
+- [#76](https://github.com/nf-core/sarek/pull/76) - Remove `runOptions = "-u \$(id -u):\$(id -g)"` in `nextflow.config` to enable `Spark` possibilities
 
 ### `Fixed`
 
diff --git a/docs/containers.md b/docs/containers.md
index 16e6291612..ddf6de64e8 100644
--- a/docs/containers.md
+++ b/docs/containers.md
@@ -21,7 +21,7 @@ For annotation, the main container can be used, but the cache has to be download
 - Contain **[Control-FREEC](https://github.com/BoevaLab/FREEC)** 11.4
 - Contain **[FastQC](http://www.bioinformatics.babraham.ac.uk/projects/fastqc/)** 0.11.8
 - Contain **[FreeBayes](https://github.com/ekg/freebayes)** 1.2.0
-- Contain **[GATK4](https://github.com/broadinstitute/gatk)** 4.1.2.0
+- Contain **[GATK4-spark](https://github.com/broadinstitute/gatk)** 4.1.4.1
 - Contain **[GeneSplicer](https://ccb.jhu.edu/software/genesplicer/)** 1.0
 - Contain **[HTSlib](https://github.com/samtools/htslib)** 1.9
 - Contain **[Manta](https://github.com/Illumina/manta)** 1.5.0
diff --git a/docs/output.md b/docs/output.md
index 4908f032ad..fd5a2c80c5 100644
--- a/docs/output.md
+++ b/docs/output.md
@@ -10,7 +10,7 @@ The pipeline processes data using the following steps:
   - [Map to Reference](#map-to-reference)
     - [BWA mem](#bwa-mem)
   - [Mark Duplicates](#mark-duplicates)
-    - [GATK MarkDuplicates](#gatk-markduplicates)
+    - [GATK MarkDuplicatesSpark](#gatk-markduplicatesspark)
   - [Base (Quality Score) Recalibration](#base-quality-score-recalibration)
     - [GATK BaseRecalibrator](#gatk-baserecalibrator)
     - [GATK ApplyBQSR](#gatk-applybqsr)
@@ -66,9 +66,9 @@ Such files are intermediate and not kept in the final files delivered to users.
 
 ### Mark Duplicates
 
-#### GATK MarkDuplicates
+#### GATK MarkDuplicatesSpark
 
-[GATK MarkDuplicates](https://software.broadinstitute.org/gatk/documentation/tooldocs/4.1.4.0/picard_sam_markduplicates_MarkDuplicates.php) locates and tags duplicate reads in a BAM or SAM file, where duplicate reads are defined as originating from a single fragment of DNA.
+[GATK MarkDuplicatesSpark](https://software.broadinstitute.org/gatk/documentation/tooldocs/current/org_broadinstitute_hellbender_tools_spark_transforms_markduplicates_MarkDuplicatesSpark.php) is a Spark implementation of [Picard MarkDuplicates](https://software.broadinstitute.org/gatk/documentation/tooldocs/current/picard_sam_markduplicates_MarkDuplicates.php) and locates and tags duplicate reads in a BAM or SAM file, where duplicate reads are defined as originating from a single fragment of DNA.
 
 This directory is the location for the BAM files delivered to users.
 Besides the duplicate marked BAM files, the recalibration tables (`*.recal.table`) are also stored, and can be used to create base recalibrated files.
@@ -510,9 +510,13 @@ For more information about how to use Qualimap bamqc reports, see [Qualimap bamq
 
 #### MarkDuplicates reports
 
-[GATK MarkDuplicates](https://github.com/broadinstitute/gatk) locates and tags duplicate reads in a BAM or SAM file, where duplicate reads are defined as originating from a single fragment of DNA.
-Duplicates can arise during sample preparation e.g.
-library construction using PCR.
+[[GATK MarkDuplicatesSpark](https://software.broadinstitute.org/gatk/documentation/tooldocs/current/org_broadinstitute_hellbender_tools_spark_transforms_markduplicates_MarkDuplicatesSpark.php), Spark implementation of [Picard MarkDuplicates](https://software.broadinstitute.org/gatk/documentation/tooldocs/current/picard_sam_markduplicates_MarkDuplicates.php)
+) locates and tags duplicate reads in a BAM or SAM file, where duplicate reads are defined as originating from a single fragment of DNA.
+
+Collecting duplicate metrics slows down performance.
+To disable them use `--skipQC MarkDuplicates`.
+
+Duplicates can arise during sample preparation _e.g._ library construction using PCR.
 Duplicate reads can also result from a single amplification cluster, incorrectly detected as multiple clusters by the optical sensor of the sequencing instrument.
 These duplication artifacts are referred to as optical duplicates.
 
diff --git a/environment.yml b/environment.yml
index 7b7b1bf897..2eebdb797c 100644
--- a/environment.yml
+++ b/environment.yml
@@ -14,7 +14,7 @@ dependencies:
   - ensembl-vep=95.2
   - fastqc=0.11.8
   - freebayes=1.2.0
-  - gatk4=4.1.2.0
+  - gatk4-spark=4.1.4.1
   - genesplicer=1.0
   - htslib=1.9
   - manta=1.5.0
diff --git a/main.nf b/main.nf
index 2ef926a2cb..2addb5e3cd 100644
--- a/main.nf
+++ b/main.nf
@@ -557,7 +557,7 @@ process BuildPonIndex {
     """
 }
 
-ch_ponIndex = params.pon_index ? Channel.value(file(params.pon_index)) : ponIndexBuilt
+ch_ponIndex = params.pon ? params.pon_index ? Channel.value(file(params.pon_index)) : ponIndexBuilt : "null"
 
 process BuildIntervals {
   tag {fastaFai}
@@ -940,15 +940,14 @@ process IndexBamFile {
 
 // STEP 2: MARKING DUPLICATES
 
-process MarkDuplicates {
+process MarkDuplicatesSpark {
     label 'cpus_16'
 
     tag {idPatient + "-" + idSample}
 
     publishDir params.outdir, mode: params.publishDirMode,
         saveAs: {
-            if (it == "${idSample}.bam.metrics" && 'markduplicates' in skipQC) null
-            else if (it == "${idSample}.bam.metrics") "Reports/${idSample}/MarkDuplicates/${it}"
+            if (it == "${idSample}.bam.metrics") "Reports/${idSample}/MarkDuplicates/${it}"
             else "Preprocessing/${idSample}/DuplicateMarked/${it}"
         }
 
@@ -956,23 +955,22 @@ process MarkDuplicates {
         set idPatient, idSample, file("${idSample}.bam") from mergedBam
 
     output:
-        set idPatient, idSample, file("${idSample}.md.bam"), file("${idSample}.md.bai") into duplicateMarkedBams
-        file ("${idSample}.bam.metrics") into markDuplicatesReport
+        set idPatient, idSample, file("${idSample}.md.bam"), file("${idSample}.md.bam.bai") into duplicateMarkedBams
+        file ("${idSample}.bam.metrics") optional true into markDuplicatesReport
 
     when: params.knownIndels
 
     script:
     markdup_java_options = task.memory.toGiga() > 8 ? params.markdup_java_options : "\"-Xms" +  (task.memory.toGiga() / 2).trunc() + "g -Xmx" + (task.memory.toGiga() - 1) + "g\""
+    metrics = 'markduplicates' in skipQC ? '' : "-M ${idSample}.bam.metrics"
     """
     gatk --java-options ${markdup_java_options} \
-        MarkDuplicates \
-        --MAX_RECORDS_IN_RAM 50000 \
-        --INPUT ${idSample}.bam \
-        --METRICS_FILE ${idSample}.bam.metrics \
-        --TMP_DIR . \
-        --ASSUME_SORT_ORDER coordinate \
-        --CREATE_INDEX true \
-        --OUTPUT ${idSample}.md.bam
+        MarkDuplicatesSpark \
+        -I ${idSample}.bam \
+        -O ${idSample}.md.bam \
+        ${metrics} \
+        --tmp-dir . \
+        --create-output-bam-index true
     """
 }
 
@@ -1132,7 +1130,7 @@ recalTableTSV.map { idPatient, idSample ->
     status = statusMap[idPatient, idSample]
     gender = genderMap[idPatient]
     bam = "${params.outdir}/Preprocessing/${idSample}/DuplicateMarked/${idSample}.md.bam"
-    bai = "${params.outdir}/Preprocessing/${idSample}/DuplicateMarked/${idSample}.md.bai"
+    bai = "${params.outdir}/Preprocessing/${idSample}/DuplicateMarked/${idSample}.md.bam.bai"
     recalTable = "${params.outdir}/Preprocessing/${idSample}/DuplicateMarked/${idSample}.recal.table"
     "${idPatient}\t${gender}\t${status}\t${idSample}\t${bam}\t${bai}\t${recalTable}\n"
 }.collectFile(
@@ -1145,7 +1143,7 @@ recalTableSampleTSV
         status = statusMap[idPatient, idSample]
         gender = genderMap[idPatient]
         bam = "${params.outdir}/Preprocessing/${idSample}/DuplicateMarked/${idSample}.md.bam"
-        bai = "${params.outdir}/Preprocessing/${idSample}/DuplicateMarked/${idSample}.md.bai"
+        bai = "${params.outdir}/Preprocessing/${idSample}/DuplicateMarked/${idSample}.md.bam.bai"
         recalTable = "${params.outdir}/Preprocessing/${idSample}/DuplicateMarked/${idSample}.recal.table"
         ["duplicateMarked_${idSample}.tsv", "${idPatient}\t${gender}\t${status}\t${idSample}\t${bam}\t${bai}\t${recalTable}\n"]
 }
@@ -1155,12 +1153,10 @@ bamApplyBQSR = bamMDToJoin.join(recalTable, by:[0,1])
 if (step == 'recalibrate') bamApplyBQSR = inputSample
 
 bamApplyBQSR = bamApplyBQSR.dump(tag:'BAM + BAI + RECAL TABLE')
-// [DUMP: recal.table] ['normal', 'normal', normal.md.bam, normal.md.bai, normal.recal.table]
 
 bamApplyBQSR = bamApplyBQSR.combine(intApplyBQSR)
 
 bamApplyBQSR = bamApplyBQSR.dump(tag:'BAM + BAI + RECAL TABLE + INT')
-// [DUMP: BAM + BAI + RECAL TABLE + INT] ['normal', 'normal', normal.md.bam, normal.md.bai, normal.recal.table, 1_1-200000.bed]
 
 // STEP 4: RECALIBRATING
 
@@ -1441,7 +1437,7 @@ bamRecal = (params.knownIndels && step == 'mapping') ? bamRecal : indexedBam
 // When starting with variant calling, Channel bamRecal is inputSample
 if (step == 'variantcalling') bamRecal = inputSample
 
-bamRecal = bamRecal.dump(tag:'BAM')
+bamRecal = bamRecal.dump(tag:'BAM for Variant Calling')
 
 // Here we have a recalibrated bam set
 // The TSV file is formatted like: "idPatient status idSample bamFile baiFile"
@@ -1517,7 +1513,8 @@ process GenotypeGVCFs {
     // Using -L is important for speed and we have to index the interval files also
     """
     gatk --java-options -Xmx${task.memory.toGiga()}g \
-        IndexFeatureFile -F ${gvcf}
+        IndexFeatureFile \
+        -I ${gvcf}
 
     gatk --java-options -Xmx${task.memory.toGiga()}g \
         GenotypeGVCFs \
diff --git a/nextflow.config b/nextflow.config
index 674bad8460..c82f5fab40 100644
--- a/nextflow.config
+++ b/nextflow.config
@@ -117,7 +117,6 @@ profiles {
     docker {
       enabled = true
       fixOwnership = true
-      runOptions = "-u \$(id -u):\$(id -g)"
     }
     singularity.enabled = false
   }