The p38 mitogen-activated protein kinases are activated in response to var-ious extracellular signals in eukaryotic cells and play a critical role in the cellular responses to these signals. The four mammalian isoforms (p38a, p38b, p38c, and p38d) are coexpressed and coactivated in the same cells. The exact role of each p38 isoform has not been entirely identified, in part due to the inability to activate each member individually.