GTP CH activity in heterozygotes nearly normal. In heteroallelic combination with Pu^rZ19 the enzyme produced is unstable at 53^oC; other combinations produce relatively heat-stable enzyme.

Embryos have completely unpigmented and weak cuticles. A portion of the homozygous embryos die before blastoderm formation with a defect in nuclear division (fewer nuclei that are misshapen, asynchronous and unevenly distributed), homozygous embryos that develop after have a segmentation defect (segment fusions and deletions).

GTP cyclohydrolase activity is reduced in the adult body and adult head but is almost normal in the prepupa of heterozygotes. Eye colour is normal in trans with Pu⁺, but is mutant in trans with a homozygous viable Pu allele.