Extracellular-signal-regulated kinases (ERKs), also known as classical MAP kinases, are part of the MAPK/ERK signaling cascade and are involved in cellular functions including regulation of proliferation, survival, and differentiation. Activation of ERK is dependent on the Ras-Raf-ERK signal transduction cascade. Malfunction of the ERK signaling pathway is implicated in several types of cancers. Currently, both small-molecule and antibody inhibitors to ERK are in clinical and pre-clinical trials as potential cancer treatments. There is hope that these may someday be used as alternative treatments for tumors that have evolved resistance to other inhibitors that act on the same signaling pathway, such as MEK inhibitors.