The basic biochemical differences which exist between cells in animal tissues are reflections of characteristic patterns of proteins. The process (es) of differentiation, therefore, may be described as the process (es) whereby cells, presumably of identical genotype, develop into phenotypically distinct entities which reflect characteristic patterns of gene activity. The differential genetic expression which initiates these different protein patterns at different stages of development and in different tissues can involve regulation of the synthesis of the characteristic protein patterns at any or all of the following steps: 1) Differential RNA transcription including the possibility of specificity and efficiency differences in RNA polymerase; 2) Non-random stabilization and maturation of potential messenger RNA within the pool of unstable heterogeneous nuclear RNA including modifications in maturation, i. e., Poly-A termination; 3) Regulation of the transport of “mature” potential messenger RNA from the nucleus to the cytoplasm including physiological and hormone mediated non random alterations in this specific transport process; 4) Differential stabilities of messenger RNA molecules in the cytoplasm including modulation involving specific polypeptide processing; 5) Modulation of the translational efficiency of messenger RNA-ribosome complexes actively involved in protein synthesis. 6) Differential stability of the protein products.