Stable and unstable genes: distribution among species and range of mutations. 
An expansion of the 'stable' gene hypothesis proposed previously is the hypothesis that there is a range of genes, some proto-oncogenes and some not, varying from those with multiple copies in the genome, wide distribution among species, and limited mutability because of a specific protective stabilizing mechanism of such genes, to those that are in single copies in the genome, unprotected, unique to one species, and highly mutable along their entire length.