The Sp1 transcription factor gene (SP1) and the 1,25-dihydroxyvitamin D3 receptor gene (VDR) are colocalized on human chromosome arm 12q and rat chromosome 7.

By means of somatic cell hybrids segregating either human or rat chromosomes, the genes encoding the transcription factor Sp1 (SP1) and the 1,25-dihydroxyvitamin D3 receptor (VDR) were both assigned to human chromosome arm 12q and to rat chromosome 7. This result implies that the locus for the clinical disorder vitamin D dependency rickets type II maps on 12q. The phenylalanine hydroxylase (PAH) and the retinoic acid receptor-gamma (RARG) genes also map on human chromosome arm 12q and rat chromosome 7, indicating that a synteny group is conserved on these chromosomes.