Expression of a constitutively active PDGFR leads to cellular transformation and suggests that, in normal cells, PDGFR activity must be tightly regulated to oppose continuous activation of its downstream effectors. PDGFR beta, for example, is known to be over-expressed in a large number of tumors, and PDGF treatment causes transformation and malignant tumors in a variety of experimental systems. One study reported the apparent expression of PDGFR alpha in nearly 100% of human lung cancer tumors examined, and reported the growth inhibition of a lung cancer cell line, A549, by Gleevec, an effect that was surmised to be mediated via PDGFR inhibition.