Positional cloning of the putative gene responsible for transient abnormal myelopoiesis (TAM) and that for multiple cartilaginous exostoses (MEX) is described. TAM is a leukemoid reaction occurring frequently in Down syndrome (DS) newborn infants and they often develop true leukemia several years later. The previous findings of "disomic homozygosity in trisomic cells" and tentative mapping of the TAM gene to 21q11.1, and an encounter of a unique DS-associated TAM patient with inv(21) (q11.1q22.13) let us start positional cloning of the TAM gene. One type of MEX is an autosomal dominant disorder and patients with MEX sometimes develop chondrosarcoma. The MEX gene has been mapped to 8q24. We encountered a sporadic case of MEX with de novo t(8q; 13q). Thus, we hypothesized that in both patients, the TAM and the MEX genes are disrupted by the structural chromosome abnormalities. For TAM, we first mapped the proximal breakpoint of inv(21) between 2 STSs using 7 cosmid clones as FISH probes that were isolated on the basis of STS markers at the 21q11.1 region, isolated their corresponding YACs, and then analyzed them. However, since YACs corresponding to 2 other STSs between the two markers could not be isolated, we carried out a chromosome walking to construct a cosmid contig between the 2 STSs. Southern analysis with a cosmid clone within the contig detected EcoRI-/HindIII extra bands on the patient's DNA. The cDNA screening and exon trapping to isolate a gene from the region are underway. Similarly, in the MEX patient we mapped the 8q breakpoint between 2 cosmid markers, then isolated YACs and cosmid subclones. By exon trapping after detection of a cosmid covering the breakpoint, an exon-like sequence was isolated. The 3'-RACE/5'-RACE revealed a novel transcript from this cosmid. Whether the transcript is the MEX gene remains to be determined.