MAPK3 and melanoma: Along with an activation of the MAPK pathway reflected by ERK1/2 phosphorylation, the analyzed melanoma cell lines not only commonly expressed the oncogenic RSK family members RSK1 and RSK2 but also exhibited marked RSK activity indicated by its activating phosphorylation at serine 380 (PS380-RSK) and threonine 359 (PT359-RSK).