Signal transduction by the JAK-STAT pathway. Cytokines signal a cell to increase transcription, and they act through a membrane-bound receptor protein. Cytokine binding to the receptor causes two receptor molecules to form a dimer, resulting in a conformational change that enables the JAK kinase to phosphorylate the receptor. This attracts the STAT protein, which in turn becomes phosphorylated and dimerizes, whereupon it enters the nucleus and activates the transcription of specific STAT-regulated genes.