We performed competitive in situ hybridization in a mediastinal germ cell tumor to identify chromosome 12 aberration(s). DNA from a mouse-human somatic hybrid cell line, containing an i(12p) as unique human material, was labeled and used as a probe. Our results confirm that the chromosome marker, observed in mediastinal germ cell tumor and cytogenetically identified as i(12p), is a true isochromosome of the short arm of chromosome 12.