The escape from immune rejection is, like the development of drug resistance in microorganisms, a multi-pathway process. Selection favors the phenotype, i.e., the cell that can grow in spite of the immune rejection process, not the underlying mechanism. Experimental evidence documents the roles of host immunosuppression (genetic or environmental), immunoresistance at the cellular level, "sneaking through" (i.e., growth of a tumor to irreversible size prior to the mobilization of an appropriate immune response), lack of antigenic recognition, and blocking enhancement type reactions.