Little is known about inherited susceptibility to spontaneous germ cells tumours in humans or other species. The Ter mutation in laboratory mice is novel in that it acts codominantly to reduce germ cell numbers on many inbred strain backgrounds and to enhance dramatically inherited predisposition to spontaneous testicular teratocarcinomas in strain 129 inbred mice. We have adopted a PCR-based, DNA pooling method for mice with 'extreme' phenotypes (small testes versus normal-sized testes) to identify a candidate linkage to the Ter locus. Two independent mapping approaches confirmed this evidence for Ter linkage near D18Mit62 on mouse chromosome 18, and suggest a possible human homologue on chromosome 5q.