Abstract: The acid .beta.-galactosidase cDNA of Portuguese Water (PW) dogs was isolated and sequenced. The entire encoding region of the gene consists of 2004 nucleotides coding a protein of 668 amino acids. Its encoding sequence indicates approximately 86.5% identity at the nucleotide level and about 81% identity at the amino acid level with the encoding region of the human acid .beta.-galactosidase gene. The deduced amino acid sequence contains a 24-amino acid putative signal sequence, 6 possible glycosylation sites, and 7 cysteine residues. A homozygous recessive mutation, causing G.sub.M1 -gangliosidosis in PW dogs, was identified at nucleotide G.sup.200 .fwdarw.A in exon 2 resulting in an Arg.sup.60 .fwdarw.His (mutation R60H) amino acid substitution. The mutation creates a new restriction enzyme site for PmlI. Genotyping 115 dog samples for this acid .beta.-galactosidase gene alteration readily distinguished affected homozygous recessive (n=5), heterozygous carriers (n=50), and normal homozygotes (n=60).