Karyotype and DNA analyses using DNA probes were carried out in a family with the Cohen syndrome. Two affected brothers had normal chromosomal constitutions. A major deletion or duplication of genomic DNA fragments hybridized with the DNA probes, pML34 at D15S9 locus and pTD3-21 at D15S10 locus, assigned on 15q11-q12 was not detected in the patients. In addition, a linkage of the syndrome to D15S9 and D15S10 loci was not observed in the family. These data suggest that a gene for the Cohen syndrome is excluded from the 15q11-q12 region, on which a gene for the Prader-Willi syndrome is assigned, and that the Cohen syndrome is distinctly different from the Prader-Willi syndrome, although clinical manifestations of the Cohen and the Prader-Willi syndromes are very similar.