p38 MAPK has 4 representative isoforms in humans, p38 alpha (Han et al. 1993), p38-beta (Jiang et al. 1996), p38-gamma (Lechner et al. 1996) and p38-delta (Hu et al. 1999). All are activated by phosphorylation on a canonical TxY motif by the dual-specificity kinase MKK6, which displays minimal substrate selectivity amongst the p38 isoforms (Zarubin & Han, 2005). p38 alpha and gamma are also activated by MKK3.