MAP3K9 and melanoma: Expression of MLK1–4 reactivated the MEK/ERK pathway in the presence of PB04 in the A375 and A2058 melanoma cell lines, confirming that MLKs directly phosphorylate MEK rather than act as a scaffold to promote RAF-mediated activation of the MEK/ERK pathway (Fig. 3c and Supplementary Fig. 2c).