While in multiple studies [19,25] comparing heme biosynthesis factors in gliomas (one of them meanwhile retracted) no significant FECH up- or downregulations were described, and the role of FECH in naturally occurring 5-ALA induced fluorescence thus remains unclear, there is convincing evidence that artificial inhibition of this enzyme actually results in higher PpIX levels due to decreased degradation [26,27,28]. Here, FECH is linked to central nervous system cancer.