It is therefore prudent to consider the two categories of patients separately. On the basis of published data, the CDR in MPD patients is flanked by D20S206 and D20S481 and the CDR in MDS/AML patients is flanked by D20S465 and D20S481 (Bench et al. , 1998a; Wang et al. , 1998). Two yeast artificial chromosome (YAC) contigs which span these CDRs have been constructed (Bench et al. , 1998a; Stoffel et al.