Rab proteins are small molecular weight GTPases that control vesicular traffic in eukaryotic cells. A subset of Rab proteins, the Rab3 proteins are thought to play an important role in regulated exocytosis of vesicles. During the large-scale sequencing analysis of a human fetal brain cDNA library, we isolated a cDNA clone encoding a novel Rab protein, which showed 99% identity with previously isolated bovine Rab3C at the amino acid level It contained four conserved motifs characteristic of the Rab3 family. RT-PCR analysis indicated that human Rab3C was expressed in the human brain, placenta, and lung. By mapping, we localized the Rab3Cgene to human chromosome 5q13. The Rab3C gene consisted of 6 exons spanning more than 310 kb of human genomic DNA. Rab3A, Rab3B, and Rab3D have been mapped to three different chromosomes, suggesting that they are not transcripts of the same gene.