TXNDC3 encodes a 588-amino acid protein with an estimated molecular mass of 67.3 kD. It contains an N-terminal domain similar to thioredoxins and a C-terminal domain composed of 3 tandemly repeated NDP kinase domains, the first of which is truncated and the last 2 complete. Its domain organization resembles that of IC1, an outer dynein arm of sea urchin axoneme. TXNDC3 also contains several potential phosphorylation sites and a PEST sequence. Gel filtration chromatography indicated that the protein migrates as a monomer. Northern blot analysis detected restricted and low-level expression of a single 2.4-kb TXNDC3 band in testis. In situ hybridization of human testis sections showed that TXNDC3 is expressed only in primary spermatocytes and round spermatids.