Globoid cell leukodystrophy is a rare autosomal recessive genetic disease caused by low levels of B-galactosidase activity, a lysosomal enzyme important in myelin metabolism. An infant rhesus monkey with this disease was diagnosed at TRPRC in 1989. In the intervening seven years, we have intentionally inbred this group of animals and in 1996 observed two additional affected infants. These two homozygous affected animals allowed us, in collaboration with Dr. David Wenger, Jefferson Medical College, Philadelphia, to identify the disease-causing mutation in the rhesus GALC gene and to unequivocally identify 22 carrier animals. The sequence of the rhesus GALC cDNA is 98% identical to the human, and the deduced amino acid sequence is 97% identical to that of humans. The organization of the human and rhesus GALC genes are also similar. The mutation responsible for GLD in these monkeys was found to consist of the deletion of the AC dinucleotide corresponding to cDNA positions 387 and 388. This mutation results in a frame shift during translation, leading to a stop codon 46 bp after the deletion. Using a PCR-based technique, we were able to identify 22 carriers and 21 noncarriers amongst the breeding group of monkeys. The carriers consist of four adult males, eight adult females, and ten juveniles. GALC activity was measured in the 2 homozygous affected, 21 normal and 20 carrier monkeys. The 2 affected infants had a GALC activity less than 2% of normal in leukocytes and cultured skin fibroblasts. The average activity for 21 normal rhesus monkeys was 0.94 nmol/hr/mg of protein, while the average for 20 carriers was 0.53. We established fibroblast and EBV-transformed lymphocyte cultures from these homozygous infants, and have initiated in vitro therapeutic studies with retroviral vectors containing the human GALC gene. Although this animal model will be useful for studies concerning the biochemistry and pathogenesis of GLD, the nonhuman primate model will be most valuable for studies of gene therapy of this and similar disorders. Several carriers are currently pregnant, and in utero diagnosis and therapy of these infants is underway.