An Hsp70 promoter drives expression of pho tagged at the N-terminal end with a 700bp HindIII fragment encoding the DNA-binding domain of Ecol\lexA.

pho^lexA.hs partially rescues pho¹

Two copies of pho^{hs.T:Ecol\lexA} produces healthy pho¹/pho¹ adults with one heat shock daily beginning 4 days after egg laying. The rescued pho¹/pho¹ males are fully fertile, but occasionally show sex comb bristles on the mesothoracic legs and excess pigmentation in the 4th abdominal tergite. The rescued pho¹/pho¹ females have severely impaired fertility that cannot be rescued with a paternal pho⁺ allele. Only 10% of embryos derived from pho¹/pho¹ females crossed to pho¹/pho¹ males hatch, even when heat shocked before blastoderm to express pho^{hs.T:Ecol\lexA}. These embryos show frequent and various segmental abnormalities but never show classical homeotic derepression phenotypes. The few larvae that hatch grow slowly and never reach the pupal stage, probably because evident head skeleton abnormalities preclude normal feeding.