Drosophila odorant receptors (Ors) are ligand gated ion channels composed of a common receptor subunit ORCO (odorant receptor co-receptor) and one of 62 ‘tuning’ receptor subunits that confer odorant specificity to olfactory neuron responses. Like other sensory systems studied to date, exposing Drosophila olfactory neurons to activating ligands results in reduced responses to subsequent exposures through a process called desensitization. We recently showed that phosphorylation of serine 289 on the common odorant receptor subunit ORCO is required for normal peak olfactory neuron responses. Dephosphorylation of this residue occurs upon prolonged odorant exposure, and underlies the slow modulation of olfactory neuron responses we term ‘slow desensitization’. Slow desensitization results in the reduction of peak olfactory neuron responses and flattening of dose-response curves, implicating changes in ORCOS289 phosphorylation state as an important modulator of olfactory neuron responses. Here, we report the ide...