The transcription start sites of many genes are associated with CpG-rich DNA regions (CpG islands) containing clusters of rare cutting, methylation-sensitive restriction enzyme sites [Bird, 1986]. To detect gene sequences from human chromosome 21, we have screened cloned DNA fragments from a chromosome 21-specific cosmid library for the presence of such restriction sites. Several DNA fragments containing rare cutter sites, including Sac II, were isolated and five of them partially characterized. The average insert size of the fragments was 38.4 kb. By using a panel of somatic cell hybrids, one insert was assigned to the distal part of region 21q21, three fragments to the region 21q22.1, and one sequence to the segment 21q22.2-22.3. Restriction mapping showed clusters of rare cutter sites in at least three of the cloned fragments, suggesting the presence of CpG islands. These fragments are thus good candidates for carriers of coding sequences.