Glycogen synthase kinase-3 is a 'black sheep' among protein kinases. Although its name suggests a primary function in intermediary metabolism, its scope is far more diverse as indicated by both genetic and biochemical evidence. Over the past five years the enzyme has emerged as an important component of transduction pathways ultimately regulating cell fate and differentiation. The molecular mechanisms underlying these functions are only just being unraveled but point to a novel mode of cellular regulation conserved in all eukaryotes.