Furthermore, we confirmed that the Ras/MEK-RSK-ABCB1 and Ras/MEK-HIF-1α-FECH axes are involved in the regulation of PpIX accumulation in human cancers, as inhibition of RSK, HIF-1α, or ABCB1 increased PpIX accumulation in multiple human cancer cell lines (Fig. 7). Here, FECH is linked to cancer.