Chromosome deletion complexes in model organisms serve as valuable genetic tools for the functional and physical annotation of complex genomes. Among their many roles, deletions can serve as mapping tools for simple or quantitative trait loci (QTLs), genetic reagents for regional mutagenesis experiments, and, in the case of mice, models of human contiguous gene deletion syndromes. Deletions also are uniquely suited for identifying regions of the genome containing haploinsufficient or imprinted loci. Here we describe the creation of new deletions at the proximal end of mouse Chromosome (Chr) 17 by using the technique of ES cell irradiation and the extensive molecular characterization of these and previously isolated deletions that, in total, cover much of the mouse t complex. The deletions are arranged in five overlapping complexes that collectively span about 25 Mbp. Furthermore, we have integrated each of the deletion complexes with physical data from public and private mouse genome sequences, and our own genetic data, to resolve some discrepancies. These deletions will be useful for characterizing several phenomena related to the t complex and t haplotypes, including transmission ratio distortion, male infertility, and the collection of t haplotype embryonic lethal mutations. The deletions will also be useful for mapping other loci of interest on proximal Chr 17, including T-associated sex reversal (Tas) and head-tilt (het). The new deletions have thus far been used to localize the recently identified t haplolethal (Thl1) locus to an approximately 1.3-Mbp interval.