Disclosed is an assay for identifying compounds that modulate a neuronal apoptotic pathway, the assay comprises: a) contacting HSP90 protein with a probe to form a probe: HSP90 complex, the probe being displaceable by a test compound; b) measuring a signal from the probe so as to establish a reference level; c) incubating the probe:HSP90 complex with the test compound; d) measuring the signal from the probe; e) comparing the signal from step d) with the reference level, a modulation of the signal indicating that the test compound binds to HSP90. Also disclosed is a probe which is labeled with a detectable label and/or an affinity tag.