The adenylate kinase1 (AK1), adenylate kinase3 (AK3), and aconitases (ACON3) genes have been assigned to chromosome 9 in man by employing an X/9 translocation segregating in man-mouse somatic cell hybrids. Segregation was controlled by taking advantage of the HAT/8-azaguanine selection-counterselection strategy directed at the X-linked HPRT locus. Assignment of AK1to chromosome 9 has suggested the assignment of the ABO blood-group locus and the nail-patella (Np) locus to 9, since both loci are linked to AK1by family studies.