Ras proteins are members of the superfamily of small GTPase. A novel human Ras-like transcript, termed RasL10B, was isolated from human blood cell cDNA library. RasL10B gene contains four exons and three introns, which encodes a 203 amino acid protein with a molecular mass of about 23.2 kDa. RT-PCR analysis showed that RasL10B is expressed extensively in human tissues. Subcellular location analysis of GFP-RasL10B fusion protein revealed that RasL10B was distributed to the cytoplasm of COS7 cells. In addition, RasL10B was expressed in E. coli Rosette (DE3) and purified to a homogenicity by Ni-NTA affinity chromatography. Finally, the mRNA levels of RasL10B were down-regulated in all human breast cancer cell lines we tested. In summary, RasL10B is a new member of Ras superfamily with tumor suppressor potential.