The amount of a polypeptide N-acetylgalactosaminyltransferase 1 when measured in blood serum. The trait is level of polypeptide N-acetylgalactosaminyltransferase 1 in blood serum (OBA_2041659). Also known as: blood serum polypeptide N-acetylgalactosaminyltransferase 1 amount.