the efficiency and duration of signal transmission is regulated by the protein kinase suppressor of Ras (KSR), which stably associates with MEK and recruits MEK to Raf.4 MEK is a key kinase that forwards upstream signals from Ras and Raf via activation of ERK.1 Once in the nucleus, ERK stimulates multiple transcription factors, resulting in the control of key cellular functions, which can lead to the progression of cancer if abnormally activated.