A cytogenetically detectable deletion, del(10) (q11.2-->q21.2), was observed in a patient with total colonic aganglionosis with small bowel involvement (TCSA), a variant of Hirschsprung disease (HSCR). A similar deletion is present in another TCSA patient (S.M. Huson, personal communication). To reveal cytogenetically undetectable deletions of chromosome 10 in further patients, we developed a strategy for mapping chromosome 10 DNA markers with respect to the observed deletions. To this end, the two chromosome 10 homologs (deleted and normal) were segregated in two distinct somatic cell hybrids obtained after fusion of the patient's fibroblasts with a Chinese hamster ovary cell line (YH21). Hybrid cells containing chromosome 10 were selected for the expression of the gene coding for the beta subunit of the fibronectin receptor (FNRB), which maps to 10p11.2, using a monoclonal antibody against FNRB. Hybrid 185.O contains the deleted chromosome, whereas hybrid 179.Q contains the nondeleted one. Southern blot and PCR analysis of DNA from these two hybrids mapped the markers RBP3H4, RET, D10S15, D10S5, D10S22, and D10S88 inside the deletion and D10S170, CDC2, EGR2, and D10S19 outside the deletion. MEN2A and MEN2B have recently been mapped within the centromeric region closely linked to RBP3 and D10S15 (which are located inside the deletion) and cosegregate with HSCR in at least two different pedigrees. Since HSCR, MEN2A, and MEN2B represent defects of neural crest cell development, we hypothesize that they originate from mutations in different genes clustered in the centromeric region of 10q.