HeJ, DBA/2J, AKRJ, IRE3(RAG) all approx 115-130 bp: CAST/ Ei 100 bp: SPRET/Ei 85 bp: C57BL/6J 60 bp. Previously identified homologs: None. Discussion: We have isolated a variable trinucleotide repeat sequence from a cosmid library prepared from the somatic cell hybrid 3E7 . Cosmids were screened with a (CAG)Io oligonucleotide, and those hybridizing were subcloned into Ml3mpl8. Subclones were rescreened with the same oligo and sequenced. One clone contained 24 tandem repeats of the trinucleotide (CAG). We have compared 131 nucleotides of flanking sequence around the (CAG)2 4 to sequences in Gen Bank and found no significant homology. A single open reading frame exists throughout the whole sequence in both strands, and in each case the repeated amino acid is alanine. PCR primers flanking the repeat amplify mouse but not human DNA. The locus maps to distal Chr 4 in a region of synteny with human Chr lp34. Expansion of trinucleotide repeats has been implicated in several human genetic disorders. So far this mechanism has not been shown to cause disorders in the mouse, but it is possible that mechanisms similar to those in humans may exist . Furthermore, distal mouse Chr 4 has been implicated in genetic susceptibility to tumor formation , and this marker will provide additional genetic mapping information in this region.