MST4/STK24 is a member of the sterile 20 (Ste20)-like kinase family that acts upstream of mitogen-activated protein kinase (MAPK) pathways and regulates c-Jun N-terminal kinase (JNK), p38 and extracellular signal-regulated kinase (ERK). Ste20-like kinases are involved in a variety of cellular processes which include gene transcription, stress responses, cytoskeletal organization and apoptosis. In response to apoptotic signals, MST4/MASK is cleaved by caspase-3 and mediates apoptosis. MST4/MASK has been shown to influence cell growth and transformation by modulating ERK (extracellular signal-regulated kinase) signaling and may play a role in prostrate cancer. MST4/MASK may also play a role in cell motility.