This t haplotype resulted from a rare recombination event between haplotype tw5 and a wild-type chromosome. DNA probes obtained by microdissection and microcloning of fragments of mouse Chromosome 17 show that this allele comprises a deletion of D17Leh54, 94, 180 and 443, and a duplication of D17Leh89, 467,and 525. This haplotype is a member of the t9 complementation group and, although derived from tw5, it is not found in wild mouse populations.