We present a high-resolution radiation hybrid map of the short arm of human chromosome 12 containing 60 loci, including 44 STSs within or closely associated with expressed sequences, 11 highly polymorphic markers, 2 anonymous sequences, 2 subtelomeric sequences, and 1 centromeric sequence. The 60 loci fell into 48 unique retention patterns, providing a comprehensive map covering the entire short arm of chromosome 12 with an average resolution of approximately 800 kb. Twenty-two unique positions were ordered in a 1000:1 framework map with an average resolution of 1.8 Mb. The proposed order is in good agreement with recently published genetic maps, high-resolution FISH maps, and YAC contigs. The noted inconsistencies involved neighboring loci permutations. Our observations further suggest the existence of chromosomal "hot spots" for breakage during irradiation. In three regions an usually high number of breaks was noted between neighboring loci compared to the physical distance derived from existing YAC contigs. Some of these hot spots seem to coincide with known chromosomal aberrations, of which at least two have been involved in the etiology of cancer.