The myc family of cellular oncogenes (c-, N- and L-myc) encode for nuclear phosphoproteins that appear to regulate cellular proliferation and differentiation during normal vertebrate development. In addition, their overexpression has been correlated with malignant transformation and apoptotic cell death. The transgenic mouse technology has been utilized to study the unique and overlapping functions of the myc family in these cellular processes in the context of the developing animal. These studies suggest that Myc oncoproteins serve important roles during normal development and that their deregulated expression is causal in the development of many malignancies. Furthermore, these experiments have provided insight into the cell type specific activities of each gene and into the unique and overlapping roles of the genes during normal development.