Mouse A9 cells containing human chromosome 7 tagged with pSV2neo were irradiated with X-rays and fused to A9 cells to isolate G418-resistant clones. From these clones, we selected radiation hybrids that contained 10-40 Mb of human DNA apparently at a single site of their genome by FISH analysis using human repetitive sequences as a probe. Then we made a panel of hybrids that contained various fragments of the 7q31-q32 region and cover its entire region altogether by PCR with STS markers of human chromosome 7. This panel is useful in chromosome transfer experiments since the dominant selective marker neo gene is attached to human DNA.