The KIT and FMS tyrosine kinase receptors, which are implicated in the control of cell growth and differentiation, stem through duplications from a common ancestor. We have conducted a detailed structural analysis of the two loci containing the KIT and FMS genes. The sequence of the approximately 90-kb KIT locus reveals the position and size of the 21 introns and of the 5' regulatory region of the KIT gene. The introns and the 3'-untranslated parts of KIT and FMS have been analyzed in parallel. Comparison of the two sequences shows that, while introns of both genes have extensively diverged in size and sequence, this divergence is, at least in part, due to intron expansion through internal duplications, as suggested by the discrete extant analogies. Repetitive elements as well as exon predictions obtained using the GRAIL and GENEFINDER programs are described in detail. These programs led us to identify a novel gene, designated SMF, immediately downstream of FMS, in the opposite orientation. This finding emphasizes the gene-rich characteristic of this genomic region.