The vertebrate eye lens has been used extensively as a model for developmental processes such as determination, embryonic induction, cellular differentiation, transdifferentiation and regeneration, with the crystallin genes being a prime example of developmentally controlled, tissue-preferred gene expression. Recent studies have shown that Pax-6, a transcription factor containing both a paired domain and homeodomain, is a key protein regulating lens determination and crystallin gene expression in the lens. The use of Pax-6 for expression of different crystallin genes provides a new link at the developmental and transcriptional level among the diverse crystallins and may lead to new insights into their evolutionary recruitment as refractive proteins.