During the past few years studies from several laboratories have utilized gene disruption approaches to define the function of members of the Stat family of transcription factors. The results have demonstrated that each family member has unique, critical, non-redundant functions in signal transduction through members of the cytokine receptor superfamily. Many of the family members mediate functions associated with innate or acquired immunity. With the availability of mice deficient in one or more of the Stats, critical experiments are possible to evaluate the roles of Stat signal transduction pathways in cellular transformation as well as evaluating their specific roles in a range of cellular responses to cytokines.