AbstractRP105 is a 105‐kDa type I membrane protein of the leucine‐rich repeat (LRR) family. Anti‐RP105 sensitizes B cells to antigen‐receptor‐mediated apoptosis, but protects B cells from radiation‐induced apoptosis and stimulates B cell proliferation. The sequence of the mouse RP105 has been reported. Here, we report the characterization of the human RP105. The 2.6‐kb cDNA encodes a protein of 661 amino acids which displays 78% homology with mouse RP105. The 22 LRR and the 9 potential N‐linked glycosylation sites within the extracellular region are conserved. While previous studies have shown that RP105 is expressed on surface IgM+IgD++ B cells in mice, human RP105 was shown to be expressed on all subsets of mature B cells and dendritic cells. Human RP105 gene was mapped to the long arm of chromosome 5, where numerous cytokines and receptors have been localized.