We were able to refine the chromosomal position of two existing marker loci, using an extended chromosome 21 somatic cell hybrid panel. The locus D21S26 mapped in the region 21q11.2-q21.1, and the locus D21S24 in 21q22.1-q22.2. Physical and genetic analysis indicated that D21S26 is tightly linked to D21S13 and D21S16, two markers previously linked to familial Alzheimer's disease.