The locus for the hereditary human Coppock-like cataract (CCL) is closely linked to a particular combination of polymorphic TaqI sites within the human gamma-crystallin gene cluster. Mapping of these sites shows that they define a 15 kb region encompassing the gamma D and psi gamma E gene. The gamma D and the psi gamma E gene were cloned from the CCL chromosome and characterized. The gamma D gene was functionally equivalent to its allele cloned from a wild-type chromosome. The CCL psi gamma E gene contains a cluster of sequence changes within and around its TATA box. Together these cause a ten-fold increase in the activity of the psi gamma E promoter, raising the level of expression of this gene to 30% of that of the gamma D gene. The predicted protein product of the psi gamma E gene is a 6 kD N-terminal gamma-crystallin fragment. Reactivation of the psi gamma E gene and concomitant overexpression of the gamma-crystallin fragment could be the cause of the Coppock-like cataract.