The hyperplastic discs (hyd) gene (formerly called l(3)c43) is located at 85E1-10 on the third chromosome of Drosophila melanogaster. It was originally identified by a temperature-sensitive mutation that causes imaginal disc overgrowth in mutant larvae raised at a restrictive temperature. Twenty new alleles of hyperplastic discs have been recovered in gamma ray, ethyl methanesulfonate and hybrid dysgenesis screens, and the molecular lesions have been identified for several of the alleles. The null phenotype appears to be lethality at or before the second instar. Adults that can be obtained in crosses of temperature-sensitive alleles maintained at permissive temperatures are sterile with one exception and have defects in germ tissue morphology. The hyperplastic discs locus has been cloned by chromosome walking from the alpha-tubulin-2 gene and encodes a 9.5-kb messenger RNA, containing a 2897-amino-acid open reading frame. Sequence analysis of HYD reveals strong similarity to a portion of the C-terminus of poly(A) binding protein and to the RAT 100-kDa PROTEIN, whose function is unknown. Developmental Northern and Western analyses show coincident accumulation of the 9.5-kb transcript and the 280-kDa protein at all stages of development, with high levels at the embryonic and pupal stages. The 280-kDa HYD protein accumulates at decreased levels in mutant alleles and at restrictive temperatures in ts alleles. Examination of relative levels of HYD protein in mutant animals support the idea that less severe mutations are those that result in disc overgrowth, while more severe mutations result in variable disc growth phenotypes.