MAPK1 and ductal breast carcinoma in situ: As expected, the mRNA expression of c-FOS, an early response gene of the MEK/ERK signaling pathway 47, was upregulated by AP20187- or bFGF-activated FGFR1 signaling in DCIS-iFGFR1 and MDA-MB-231 cells, respectively, and inhibited by PD0325901, a MEK inhibitor that also prevents ERK1/2 activation, in both types of cells (Fig.