LSP1 is a 52 kDa leukocyte-specific protein expressed in B cells, T cells, macrophages and neutrophils. The protein binds F-actin through its C-terminal actin-binding repeat and regulates cytoskeletal-related cellular movement such as polarization, chemotaxis and locomotion. LSP1 is a phospho-protein and has been identified as a major substrate of MAP kinase and PKC in lymphocytes and neutrophils.