The Hsp90 inhibitor IPI-504 rapidly lowers EML4-ALK levels and induces tumor regression in ALK-driven NSCLC models..     ///    We find EML4-ALK to be more sensitive to Hsp90 inhibition than either HER2 or mutant epidermal growth factor receptor (EGFR) with an inhibitory concentration (IC)(50) for protein degradation in the low nanomolar range.     ///    This degradation leads to a potent inhibition of downstream signaling pathways and to the induction of growth arrest and apoptosis in cells carrying the EML4-ALK fusion.     ///    "To generate a causative link between the expression of EML4-ALK and sensitivity to IPI-504, we introduced an EML4-ALK cDNA into HEK293 cells and show that the expression of the fusion protein sensitizes cells to IPI-504 both in vitro and in vivo. "