Ten new and seventeen previously reported Enhanced S Cone Syndrome (ESCS) subjects were used to search for genetic heterogeneity. All subjects were diagnosed with ESCS on the basis of clinical, psychophysical and/or electroretinography testing using published criteria. Mutation analysis was performed on the NR2E3 nuclear receptor gene by single strand conformation analysis and direct sequencing, which revealed either homozygous (N=13) or compound heterozygous (N=11) mutations in 24 subjects (89%), heterozygous mutations in 2 subjects (7%) and no mutations in 1 subject (4%). Fifteen different mutations were identified, including six not previously reported. The subject (Patient A) with no detected NR2E3 mutation had features not usually associated with ESCS, in particular moderate rod photoreceptor function in peripheral retina and an abnormally thick retinal nerve fibre layer. Mutation analysis of the NRL, CRX, NR1D1 and THRB genes in this individual revealed a heterozygous one base-pair insertion in exon 2 of the NRL gene, which results in a predicted truncation of the NRL protein. Loss-of-function NRL alleles have not been described previously in humans, but since the same mutation was present in unaffected family members, it raises the possibility that the abnormal ESCS phenotype in Patient A may result from a digenic mechanism, with a heterozygous NRL mutation and a mutation in another unknown gene.