APOL1 and kidney disorder: Various theories for this atypical phenotypic penetrance have been put forth and the most persuasive is that, similar to initial theories for MYH9 and African-American glomerulosclerosis, perhaps the APOL1 G1 and G2 alleles do not cause but rather predispose to kidney disease, and a second gene-gene or gene-environment interaction (as yet unknown) is required for the phenotype.