Leber congenital amaurosis (LCA) is a congenital retinal dystrophy characterized by severe visual loss in infancy and nystagmus. Although most often inherited in an autosomal recessive fashion, rare individuals with mutations in the cone‐rod homeobox gene, CRX, have dominant disease. CRX is critical for photoreceptor development and acts synergistically with the leucine‐zipper transcription factor, NRL. We report on the phenotype of two individuals with LCA due to novel, de novo CRX mutations, c.G264T(p.K88N) and c.413delT(p.I138fs48), that reduce transactivation in vitro to 10% and 30% of control values, respectively. Whereas the c.413delT(p.I138fs48) mutant allows co‐expressed NRL to transactivate independently at its normal, baseline level, the c.G264T(p.K88N) mutant reduces co‐expressed NRL transactivation and reduces steady state levels of both proteins. Although both mutant proteins predominantly localize normally to the nucleus, they also both show variable cytoplasmic localization. These observations suggest that some CRX‐mediated LCA may result from effects beyond haploinsufficiency, such as the mutant protein interefering with other transcription factors' function. Such patients would therefore not likely benefit from a simple, gene‐replacement strategy for their disease. © 2010 Wiley‐Liss, Inc.