Normal and tumor DNAs from 112 patients were screened for LOH on 9p with 12 microsatellite markers and on proximal 9q with six markers (Fig. 1). For this study, the q arm markers served to identify tumors with deletion of p only or separate deletions on p and q. Eleven tumors showed deletion only on 9p. and eight tumors revealed discrete deletions of 9p and 9q separated by a region of retention. Of particular importance was a tumor (T46) which showed retention of heterozygosity at all informative loci on 9p and 9q, except at the IFNa locus at 9p2l-22 where clear LOH was observed (Figure 2A).