Members of the Myc family of transcription factors are key regulators of ce
ll proliferation, and excessive levels of Myc lead to tumor formation. Mad
family proteins are related to Myc, but they antagonize the oncogenic activ
ity of Myc in cell-culture assays. Here, we examine current models of Mad f
unction and the relationship between Mad and Myc in cell proliferation, dif
ferentiation and tumorigenesis.