Spontaneous Rhabdomyosarcomas in (A/J x CBA/J)Fl Mice.

Spontaneous rhabdomyosarcoma is rare in laboratory rodents. Incidence was estimated as 2.4/100,000 in BALB/c mice; incidence of <0.5% for rodents is generally accepted. We describe spontaneous rhabdomyosarcoma in 2 aged, female ([A/J x CBA/ J]F1) mice. Part of a small breeding colony, these mice were experimentally naive; incidence was estimated as 12.5%. Tumors were located in the muscles of the thoracic wall and muscles of the dorsolumbar region. Cross-striations were detected, using phosphotung- stic acid hematoxylin stain; presence of cross-striations within the neoplasm are diagnostic for rhabdomyosarcoma.