Exon 1 of ref(2)P+++.O1.cCMa has been replaced with exon 1 of ref(2)Pdpp'.Pa.cCMa. This introduces the d, p and p' polymorphisms into the PB1 domain of the ref(2)P sequence. The d polymorphism consists of amino acid residues Q28 and N29 (in the PB1 domain) being replaced by a single G residue. The p polymorphism consists of amino acid residue I32 being replaced by a V residue. The p' polymorphism consists of amino acid residue Q43 being replaced by a L residue.