Heat shock transcription factor HSF4 is necessary for ocular lens development and fiber cell differentiation. Mutations of the human HSF4 gene have been implicated in congenital and age-related cataracts. Here, we show that HSF4 activates transcription of genes encoding crystallins and beaded filament structural proteins in lens epithelial cells. Five missense mutations that have been associated with congenital cataract inhibited DNA-binding of HSF4, which demonstrates the relationship between HSF4 mutations, loss of lens protein gene expression, and cataractogenesis. However, two missense mutations that have been associated with age-related cataract did not or only slightly alter HSF4 activity, implying that other genetic and environmental factors affect the functions of these mutant proteins.