Importantly, since our yeast model system revealed that WDR36 mutations have biologic consequences only when simultaneously present with variation of a second gene (e.g., STI1), we hypothesize that glaucoma patients who are WDR36 heterozygotes also have DNA alterations at other modifying loci and that the presence of the additional alterations is necessary for these individuals to develop the disease. The gene discussed is STIP1; the disease is glaucoma.