are therefore useful for studying the nature and expression of this normal cell gene and in elucidating the physiological role of its products. RNA tumor viruses perhaps offer us one of the most complete sets of biochemical reagents and biological responses for examining gene regulation in vertebrates and for studying the consequences of aberrant gene regulation on cell growth in tissue culture and in animals. Further­ more, there is an increasing conviction that virogenes play an important role in normal development and/or differentiation (RISSER, STOCKERT and OLD, 1978). Consequently, there is a growing feeling that DNA proviruses are altered viro­ genes and are capable of interfering with normal development or differentiation, causing reprogrammed growth or the incapacity to specialize.