Phosphorylation of CEBPB (C/EBP-beta) serine residue S321 by ERK1/2-activated RSK1, RSK2 or RSK3, downstream of activated RAS, is necessary for the relief of CEBPB autoinhibiton (Lee et al. 2010). Phosphorylation on other sites may also be involved in CEBPB activation.