Transmission ratio distortion (TRD) is a property of complete t haplotypes which results in the preferential transmission of the t haplotype chromosome from heterozygous t/+ males to the majority of the offspring. A candidate gene for one of the primary genetic elements in TRD, the t complex responder locus has recently been suggested to be Tcp-10bt. There are multiple, functional Tcp-10t genes, but genetic data suggest the presence of the Tcp-10at gene alone is compatible with normal transmission ratios. Here we present the complete sequence and genomic structure of the Tcp-10at gene which is compared with sequence data from a number of cDNAs and genomic subclones representing all active Tcp-10t family genes. A detailed table of all sequence variants discovered in the course of our investigation is presented, and we have clarified the extent of 5' untranslated alternative splicing patterns exhibited by this gene family. A 60 base pair (bp) in-frame deletion from the 5' end of exon 3 of the Tcp-10at gene is also presented and compared with the equivalent region of Tcp-10bt and Tcp-10ct. A search of the University of Edinburgh database has revealed a significant homology between the Tcp-10bt open reading frame and several cytosolic filament proteins. Interestingly, the region of homology is involved in the deletion from the Tcp-10at gene.