The receptor-associated JAK protein kinases and their substrates, the STAT transcriptional activators, transmit signals following cytokine and growth factor binding to receptors expressed on the cell surface, to result in specific transcriptional and cellular responses. Over the last two decades, the field has progressed from identification of the individual components through to an understanding of the activation and deactivation mechanisms, and the complex structural detail of the proteins involved. We now know that these pathways are important in many biological processes, including...