The sev^B4 allele is temperature sensitive; at 22.7°C, flies that are homozygous for it develop normal R7 photore-ceptors, but at 24.3°C, they fail to develop these photo-receptors. sos^2A is a recessive, loss-of-function mutation in the son of sevenless(sos) gene. Flies with the genotype sev^B4/sev^B4;  sos 2A/ + fail to develop R7 photoreceptorsif they are raised at 22.7°C. Therefore, sos2Aacts as a dominant enhancer of the sevB4 mutant phenotype at this temperature. Based on this observation, where is the protein product of the wild-type  sosgene—called SOS—likely to act in the pathway for R7 differentiation?