Serine/threonine-protein kinase A-Raf or simply A-Raf is an enzyme that in humans is encoded by the ARAF gene. A-Raf is a member of the Raf kinase family of serine/threonine-specific protein kinases. Compared to the other members of this family (Raf-1 and B-Raf), very little is known about A-Raf. It seems to share many of the properties of the other isoforms, but its biological functions are not as thoroughly researched. All three Raf proteins are involved in the MAPK signaling pathway. There are several ways A-Raf is different from the other Raf kinases. A-Raf is the only steroid hormone-regulated Raf isoform. In addition, the A-Raf protein has amino acid substitutions in a negatively charged region upstream of the kinase domain (N-region). This could be responsible for its low basal activity.