Our research establishes genetic linkage between the hph-1 mutation and the GTP-CH I structural gene. Our results indicate that these two loci are within an 8 cM region with 95% confidence. This finding lends additional support for the use of the hph-1 mouse mutant as a bona fide model system for the human disorder GTP-CH I to further our understanding of the molecular mechanisms involved in the disease pathology of GTP-CH I deficiency.