Williams syndrome (WS) is a complex neurodevelopmental disorder arising from a microdeletion at Chr band 7q11.23, which results in a hemizygous condition for a number of genes. Within this region we have completely characterized 200 kb containing the genes LIMK1, WBSCR1, and RFC2. Evidence was also found for WBSCR5 in this region, but not the previously proposed genes WSCR2 and WSCR6. The syntenic region in mouse was also sequenced (115 kb) and characterized, and a comparative sequence analysis with a percent identity plot (PIP) easily allowed us to identify coding exons. This genomic region is GC rich (50.1% human, 49.9% mouse) and contains an unusually high abundance of repetitive elements consisting primarily of Alu (45.4%, one of the highest levels identified to date) in human, and the B family of SINES (30.6% of the total sequence) in mouse. WBSCR1 corresponds to eukaryotic initiation factor 4H, identified in rabbit, and is herein found to be constitutively expressed in both human and mouse, with two RNA and protein products formed (exon 5 is alternatively spliced). The transcription pattern of WBSCR5 was also examined and discussed along with its putative amino acid sequence.