Su(s) localizes to a subset of heat shock (HS) puffs. Chromosomes from heat-shocked (37°C) larvae were probed to detect Su(s) (red) and Ser2∼P (green) and stained with DAPI (4′,6-diamidino-2-phenylindole; blue). Arrows indicate the cytological positions of heat shock loci. Chromosomes were obtained from the following stocks: su(s)+ (A, B, and D); su(s)+ transformant line with an Hsp70 promoter insertion at 23A (C); su(s) null mutant (E); and transformant lines carrying su(s)ΔARM and su(s)ZF mutant transgenes, respectively, in the su(s) null mutant background (F and G). Su(s) was detected at the endogenous 87C and 67B heat shock puffs and at the site of an ectopic Hsp70 promoter insertion at 23A. Su(s) was present at 87C in the su(s)ΔARM mutant but was absent from this region in the su(s) null and su(s)zf mutants.