tokens: Here we report ( 1 ) identification of another mutation , on a haplotype 9 chromosome , which converts codon 65 from isoleucine ( ATT ) to threonine ( ACT ) , ( 2 ) expression analysis of the I65T mutation in COS cells demonstrating 75 % loss of both immunoreactive protein and enzyme activity , and ( 3 ) expression analysis of the most prevalent PKU allele ( M1V ) in eastern Quebec , showing nondetectable levels of PAH protein and activity , a finding compatible with a mutation in the translation initiation codon .