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