Phenylketonuria (PKU) is an inborn error of metabolism that is inherited in an autosomal recessive manner. It arises from a deficiency of phenylalanine hydroxylase, which is responsible for converting phenylalanine to tyrosine and thereby hastening its catabolism. To produce mouse models for the study of PKU, male mice were mutagenized with ethylnitrosourea and their progeny were screened for the elevated phenylalanine levels characteristic of phenylalanine hydroxylase deficiency. Of three mutant alleles recovered, two (Pah(enu1) and Pah(enu2)) were characterized previously and shown to be missense mutations. Sequencing of phenylalanine hydroxylase cDNA from the third mutant allele, Pah(enu3), revealed that two differently sized transcripts were being produced. These transcripts contained either a 5-nucleotide insertion or a 5-nucleotide deletion and both of these modifications occurred at the same location, the exon 11-exon 12 junction. Sequencing of the exon 11-intron 11 boundary revealed a T --> G transversion in the invariant GT dinucleotide of the wild-type 5' splice donor site. The analogous human Pah mutation would be called c.1199 + 2T > G. Sequence analysis also revealed two cryptic splice donor sites, upstream and downstream of the wild-type splice site, that appear to be used when the wild type is ablated and to thereby yield the observed differently sized transcripts. The 5-nucleotide insertion and the 5-nucleotide deletion are both predicted to cause frame shifting in exon 12 and exon 13, leading to premature termination.