diff --git a/eth/chains/mainnet/__init__.py b/eth/chains/mainnet/__init__.py
index e303740923..cf77195b47 100644
--- a/eth/chains/mainnet/__init__.py
+++ b/eth/chains/mainnet/__init__.py
@@ -44,6 +44,27 @@
 )
 
 
+def validate_header_is_on_intended_dao_fork(support_dao_fork: bool,
+                                            dao_fork_at: BlockNumber,
+                                            header: BlockHeaderAPI) -> None:
+
+    # The special extra_data is set on the ten headers starting at the fork
+    extra_data_block_nums = range(dao_fork_at, dao_fork_at + 10)
+
+    if header.block_number in extra_data_block_nums:
+        if support_dao_fork and header.extra_data != DAO_FORK_MAINNET_EXTRA_DATA:
+            raise ValidationError(
+                f"Block {header!r} must have extra data "
+                f"{encode_hex(DAO_FORK_MAINNET_EXTRA_DATA)} not "
+                f"{encode_hex(header.extra_data)} when supporting DAO fork"
+            )
+        elif not support_dao_fork and header.extra_data == DAO_FORK_MAINNET_EXTRA_DATA:
+            raise ValidationError(
+                f"Block {header!r} must not have extra data "
+                f"{encode_hex(DAO_FORK_MAINNET_EXTRA_DATA)} when declining the DAO fork"
+            )
+
+
 class MainnetDAOValidatorVM(HomesteadVM):
     """Only on mainnet, TheDAO fork is accompanied by special extra data. Validate those headers"""
 
@@ -53,23 +74,11 @@ def validate_header(self,
                         check_seal: bool=True) -> None:
 
         super().validate_header(header, previous_header, check_seal)
-
-        # The special extra_data is set on the ten headers starting at the fork
-        dao_fork_at = self.get_dao_fork_block_number()
-        extra_data_block_nums = range(dao_fork_at, dao_fork_at + 10)
-
-        if header.block_number in extra_data_block_nums:
-            if self.support_dao_fork and header.extra_data != DAO_FORK_MAINNET_EXTRA_DATA:
-                raise ValidationError(
-                    f"Block {header!r} must have extra data "
-                    f"{encode_hex(DAO_FORK_MAINNET_EXTRA_DATA)} not "
-                    f"{encode_hex(header.extra_data)} when supporting DAO fork"
-                )
-            elif not self.support_dao_fork and header.extra_data == DAO_FORK_MAINNET_EXTRA_DATA:
-                raise ValidationError(
-                    f"Block {header!r} must not have extra data "
-                    f"{encode_hex(DAO_FORK_MAINNET_EXTRA_DATA)} when declining the DAO fork"
-                )
+        validate_header_is_on_intended_dao_fork(
+            self.support_dao_fork,
+            self.get_dao_fork_block_number(),
+            header
+        )
 
 
 class MainnetHomesteadVM(MainnetDAOValidatorVM):