Abstract. The mAb lan3-6 recognizes a cytosolic antigen which is selectively expressed in the growth cones and axons of a small subset of peripheral sensory neurons fasciculating in a single tract common to all hirudinid leeches. We have used this antibody to clone a novel EF-hand calcium-binding protein, calsensin, by screening an expression vector library. A full-length clone of 1.1 kb identified by the antibody was isolated and sequenced. In situ hybridizations with calsensin probes and antibody staining using new polyclonal antisera generated against calsensin sequence demonstrate that calsensin indeed corresponds to the lan3-6 antigen. Calsensin consists of 83 residues with a calculated molecular mass of 9