[15] Anathrax lethal toxin (LeTx) has a similar activity profile to PD98059 in cancer cell lines [37] and is known to inactivate MAPK signaling pathways by cleaving the amino termini of MAP2K1, MAP2K2, MAP2K3, MAP2K4, MAP2K6, and MAP2K7 (reviewed in ref [16]). Bromberg-White et al. [16] used an in vivo murine model of retinal vascular development to examine the effects LeTx on nascent retinal vascular networks. Here, MAP2K4 is linked to cancer.