cancer a potentially preventable disease current epidemiologic and carcinogenesis data are lending credence to the hypothesis that individual cancers have specific causes because of this most cancers are at least potentially preventable primary prevention with removal of specific etiological agents is the best means of prevention such as elimination of tobacco smoke short of knowing the specific etiology new chemoprevention techniques are now being studied evaluating hormonal agents antiinflammatory drugs and the structural analogs of vitamin a significant similarity and dissimilarity in homologous proteins common practice emphasizes significant sequence similarities between different members of protein families these similarities presumably reflect on evolutionary conservation of structurally and functionally essential residues the nonconserved regions on the other hand may be either selectively neutral or differentiated we propose several distributional sequence statistics eg clustering of charged residues compositional biases and repetitive patterns as indicators of differentiation events these ideas are illustrated with various examples including comparisons among g proteincoupled receptors herpesvirus proteins and gtpaseactivating proteins