A diagram of the Ras/Raf/MEK/ERK pathway, showing the possible distinct functions for ERK1 and ERK2. A, B and C represent hypothetical downstream effectors. Proliferation and Ras-induced transformation require ERK2 but not ERK1. ERK1 appears to act antagonistically to ERK2 to compete for MEK, resulting in a dampening of ERK2 signaling. It is currently not known whether ERK1 and ERK2 have different substrates.