NGF and pheochromocytoma: K252a is a serine/threonine protein kinase inhibitor (IC50s of 10 to 30 nM) that has been shown to block neuronal differentiation in rat pheochromocytoma cells and is also a potent inhibitor (IC50 of 3 nM) of tyrosine protein kinase activity of the nerve growth factor trk oncogene [40].