Clonal Origin of Skin and Bone Tumors Produced by Repeated Beta‐irradiation in Mosaic Cell Mice

Clonal origin of skin and bone tomors produced by repeated beta‐irradiation was determined by using mice with cellular mosaicism created by random X‐chromosome inactivation, on the basis of phosphoglycerate kinase‐1 (PGK). The backs of female C3H/He (Pgk‐1 a /Pgk‐1 b ) mice were exposed to beta rays from 90 Sr‐ 90 Y at a dose of 3 Gy per exposure 3 times weekly until tumors appeared. The cumulative tumor incidence reached 100% 500 days after the beginning of irradiation, as determined by the Kaplan‐Meier method. All 8 tumors examined were of a single PGK phenotype: 5 squamous cell carcinomas and 2 osteosarcomas of A‐type, and 1 squamous cell carcinoma of B‐type. The absence of double PGK phenotype (AB‐type) tumors indicated the monoclonal origin of the tumors produced by repeated irradiation.