Quantification of staining intensities for the SG3 and SG10 per–lacZ transformants. Three observers were asked to rank (X-gal-mediated) staining intensities of fly brains sectioned at ZT24 and ZT12 on the same day (see Materials and Methods). The intensity for a given pair of flies (with the sameper genotype) was classified as “higher at lights on” (ZT24), “higher at lights off” (ZT12), or “equal.” For the per⁺ SG3/SG3 andper⁺ SG10 transgenics, five pairs of flies were scored, and for per⁰¹SG3/SG3, four pairs (resulting in a total of 15 classifications for theper⁺ genetic background and 12 classifications in the per⁰¹background). The majority of fly pairs from all genotypes showed equal staining at both time points or higher staining at lights off (in contrast to wild-type PER or BG- and XLG-per–lacZfusion proteins, which stain most intensely around lights on) (Zerr et al., 1990; present study).