SummaryTwo hybrid cell lines, whose only human material was a portion of the X translocated on to a mouse chromosome, have been characterized by cytogenetics,in situhybridization and Southern blotting. In one hybrid (HORL911R8B) the region Xpter to Xq2(2–4) was identified. In the other (PIP) the single human fragment was found to contain sequences from two separate X chromosomal regions (corresponding approximately to Xp11.4–Xp22.1 and Xq26–Xqter). These two hybrids in combination with a third (WAG 8) retaining Xqter to Xp21 as a human X‐autosome translocation chromosome, form a mapping panel for rapid subregional assignments to the human X chromosome. This mapping panel has been used to provide information about the order of DNA sequences derived from the X chromosome and to provide an assignment for an anonymous DNA segment, M201γ, to Xp11.