AbstractHuman neuroblastomas show a high incidence of deletions in the distal region of the short arm of chromosome 1. In pursuit of a molecular analysis of these deletions, we have generated a microclone bank from microdissected 1p35‐pter chromosomal fragments. To allow a rapid localization of the microclones, we have also generated a panel of (human X mouse) hybrid cell lines through microcell‐mediated chromosome transfer. The hybrid cells contained different portions of the human chromosome 1 on a murine background. A total of 20 randomly chosen single or low‐copy microclones were localized by Southern analysis on DNA of the hybrid panel: All probes were derived from chromosome 1. Sixteen mapped in region 1p36.I‐pter, two in 1p22‐p36.1, and another two in 1 cen‐qter. The mapping of ten of these microclones was further refined by in situ hybridization. Cells of the neuroblastoma line GI‐ME‐N carry two types of chromosome 1, one cytogenetically normal and another with a translocation reported to be in 1p36.2, i.e., a t(1;?)(p36.2;?) marker. Using cell hybridization, we separated the two chromosome 1 types of GI‐ME‐N into different hybrid cell clones. Southern hybridization of three microclones from distal 1p to DNA of the hybrid cell clones revealed that the breakpoint in the translocated chromosome 1 wa