The RSK (ribosomal S6 kinase) family comprises growth factor-regulated serine/threonine kinases, known also as p90(rsk). RSK1 contains 2 nonidentical kinase catalytic domains and phosphorylates various substrates, including members of the mitogen-activated kinase (MAPK) signalling pathway. Moller described the cloning and characterization of 3 genes encoding RSKs, and HU1 (also named RPS6KA1, or RSK1) cDNA encodes a predicted 735-amino acid protein containing 2 distinct consensus ATP-binding site sequences. Northern blot and RNase protection analyses detected an approximately 3.5-kb HU1 transcript in lymphocytes, skeletal muscle, liver, and adipose tissue (1). Zeniou determined the expression of the RSK1, RSK2, and RSK3 genes in various hum