Signaling molecules and receptors work in different combinations to regulate the diverse cellular functions. Signals from different receptors, each stimulated by their ligand, can converge to activate a common downstream effector — a protein that triggers a specific cellular response. Individual signals from G-protein coupled receptors, tyrosine kinase receptors, or integrins, can activate Raf, a Ras effector and trigger the mitogen-activated protein or MAP kinase pathway. Signals from an activated receptor can also diverge to activate multiple downstream pathways. For example, insulin binding to its receptor can trigger both phosphoinositide 3 kinase and Ras pathways. Pathways cross-talk when molecules of one pathway are either shared between multiple pathways or molecules of one pathway are modified by enzymes of another. In the cyclic AMP pathway protein kinase A, PKA activates the transcription factor, cyclic AMP response element-binding protein by phosphorylation. Additionally, through crosstalk, PKA can also block the signal transduction through the MAP kinase pathway.