The human selenocysteine tRNA gene (TRSP) has been localized on chromosome 19q13.2-->q13.3 by in situ hybridization and ordered with respect to other genes and anonymous DNA markers in this region by linkage analysis in the forty CEPH pedigrees. These loci span only 10 cM in males and about 30 cM in females. The order of the loci is cen ... D19S7-D19S9-D19S47-CYP2A-CYP2F1-APOC2++ +-(TRSP, CKM). CYP2B flanks the CYP2A and CYP2F1 loci, but it cannot be determined whether it is proximal or distal to the other two cytochrome P450 loci with respect to the centromere.