MARCKS (myristoylated alanine-rich C kinase substrate, 32 kDa) and its 20 kDa brother MARCKS-related protein (MRP) are abundant, widely distributed proteins unusually rich in alanine and glutamic acid, and with lysines, serines and phenylalanines concentrated in a compact "effector domain" (ED) near the middle of the sequence. Its conformation in solution appears to be labile, with little evidence for definite secondary structure. MARCKS (and MRP) interact inter alia with lipid bilayer membranes (via the myristoyl group and the ED), with protein kinases (which phosphorylate the serines in the ED), and with calmodulin (via the ED); synergies between these diverse interactions present an unusually rich array of possibilities for a variety of regulatory rôles. The proteins appear to be essential for controlling cell shape changes, possibly via involvement in cytoskeleton-membrane linkage. MRP deficiency leads to neural tube defects in brain development; MARCKS overexpression strongly depresses the proliferation of cancer cells.