A hybrid dysgenesis-induced mutation, enhancer of rudimentaryp1 (e(r)p1), is a recessive enhancer of a weak rudimentary mutant phenotype in Drosophila melanogaster. The e(r) gene was cloned using P element tagging and localized to region 8B on the X chromosome. It encodes a 1.0-kb and a 1.2-kb transcript. The 1.0-kb transcript is present in both adult males and females, while the 1.2-kb transcript is predominantly found in females. The difference in the lengths of the two e(r) transcripts is caused by two different polyadenylation sites spaced 228 bp apart. The amounts of both of these transcripts are drastically reduced in the e(r)p1 mutant. The P element in e(r)p1 is inserted in the 5'-untranslated leader region near the start of transcription. It may be producing its effect by suppressing transcription and/or by providing transcription termination and polyadenylation signals. The putative e(r) protein is 104 amino acids in length and bears no striking resemblance to protein sequences in GenBank or PIR. While its biochemical function is unknown at this time, sequence analysis indicates that the e(r) protein is highly conserved and, presumably, functionally very important. The amino acid sequences of the D. melanogaster and the Drosophila virilis proteins are 95% identical.