Modifier of rudimentaryp1 (mod(r)p1) is a hybrid dysgenesis-induced mutation isolated as a suppressor of rhd1, a hypomorphic rudimentary(r) allele in Drosophila melanogaster.mod(r)p1 has opposite effects on two of the rudimentary mutant phenotypes. It suppresses the wing truncation associated with hypomorphic r alleles, which was the phenotype used to isolate it. On the other hand, it does not suppress the sterility of r females and in fact decreases the fertility of wild-type females. This infertility is associated with a drastic decrease in r expression in mod(r)p1 females. P elementagging was used to clone the mutant allele, mod(r)p1. Subsequently, 28 kb of genomic DNA encompassing the wild-type mod(r) gene in the chromosomal region 1B was cloned. mod(r) encodes a 1.3 kb transcript which is not detected in the mod(r)p1 mutant. The sequences of mod(r) cDNA clones reveal that the gene encodes a protein of 200 amino acids in length. When compared to sequences in GenBank, the amino acid sequence did not reveal any long sequences similarities. However, the structure of the protein reveals similarities to known transcription factors. The N-terminal half of the protein is very acidic, whereas the C-terminal half is basic. The basic domain suggests a possible DNA-binding domain, while the acidic domain suggests a transcriptional activation domain. Consistent with this possibility is the fact that mod(r) acts through the 5' control region of the rudimentary gene to control its expression.