The gene A4GALT encodes 4-α-galactosyltransferase, the enzyme that synthesizes P1 and Pk antigens. P1 antigen is synthesized on paragloboside and Pk antigen on lactosylceramide. Changes in exon 2a distinguish P1 and P2 phenotypes. Differences from reference allele A4GALT*P1.01 (accession number GU902278) are given below. A nucleotide change of C>T (ACG>ATG) in exon 2a of A4GALT introduces an open reading frame in transcripts that include exons 1 and 2a. This is associated with fewer enzyme-encoding transcripts (comprising exon 3) in the presence of the P2 allele but it is unknown how this transcriptional regulation occurs.