xperiment 1: measure per-luc gene expression while keeping lights off in the preceding section, you demonstrated that per-luc transgenic flies have a predictable pattern of light emissions that reflects the activity of the per gene. however, is it possible that the oscillations in gene expression occur because the flies are responding to the 12-hours-light/12-hours-dark cycle that we imposed? do flies and other organisms depend on external light sources to maintain these rhythms? let's test these questions. methods 1. for 5 days, observe light emissions during normal 12-hours-light/12-hours-dark cycle (lights on: 6:00 a.m. to 6:00 p.m.; lights off: 6:00 p.m. to 6:00 a.m.). 2. on day 6, turn off lights. 3. measure light emissions in constant darkness for 5 days. average the data from 20 flies.