The physiological role of each individual STAT protein is now being examined through the study of ‘knockout’(KO) mice, harboring a null allele for the particular gene. In contrast to other STATs deficient mice that are born alive, STAT3-deficient mice die during early embryogenesis. However, the role of STAT3 in adult tissues can be assessed by utilizing the Cre-loxP recombination system to ablate the gene in later life. Analyses of tissue-specific STAT3-deficient mice indicate that STAT3 plays a crucial role in a variety of biological functions including cell growth, suppression and induction of apoptosis, and cell motility.