The biology of tumour suppression.

Evidence is presented for the view that suppression of malignancy in hybrids between malignant and normal cells is achieved in one of two ways. The normal cell either imposes its own pattern of terminal differentiation on the hybrid or it complements a genetic block in the differentiation programme of the malignant cell. The progressive multiplication of malignant cells in the animal is seen as a secondary consequence of blocked differentiation. Karyological examination of hybrids in which malignancy is suppressed is entirely consistent with the view that programmes of differentiation may be governed in a pleiotropic fashion by single genes.