Identification of X-box regulated genes facilitated the cloning of the C. elegans dye filling defective gene, dyf-5. M04C9.5 in C. elegans and its orthologs in C. briggsae (CBG22182) and C. remanei (Cr_M04C9.5) all have X-box motifs in their promoters. The C. elegans candidate gene M04C9.5 matches the genetic position of dyf-5. Sequencing of M04C9.5 in the dyf-5 strain revealed that it carries a G→A point mutation in its second coding exon, which generates a nonsense mutation and, therefore, causes a premature termination in translation. Numbers next to X-box motifs are their HMM scores. This figure was drawn using the Generic Genome Browser .