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The Drosophila transformer‐2 gene (tra‐2) is required for female sex determination in somatic cells and for spermatogenesis in male germ cells. We studied the organization of the tra‐2 gene and characterized the transcripts in wild type and mutant animals. Two transcripts are detected in males and females; they differ in their abundance and in the presence (minor transcript Tmin) or absence (major transcript Tmaj) of one exon. Two other transcripts are present only in male germ cells. One of these is rare (msTmin) and represents a spliced form of the other, more abundant transcript (msTmaj). The transcript Tmaj encodes a protein of 264 amino acids, whereas transcripts Tmin and msTmaj encode proteins that are truncated at the N‐terminus. All three putative proteins contain a stretch of approximately 90 amino acids, the ribonucleoprotein motif (RNP motif), which shows similarity to a variety of different ribonucleoproteins. Transformation studies reveal that a cDNA corresponding to the transcript Tmaj can provide all the functions for female sex determination and male fertility. Surprisingly, a cDNA corresponding to the transcript msTmaj could only supply some female sex‐determining function, but was unable to restore fertility in mutant males. Sequence analysis of two temperature‐sensitive mutations provides evidence that the RNP motif represents an important functional domain of the tra‐2 protein.

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