CD24 is expressed in B cells until their terminal differentiation into plasma cells. It has been used widely as a marker to fractionate different B-lineage differentiation stages. CD24 has been found on a variety of other hematopoietic and neural cell types and is now classified as a costimulatory molecule. The CD24 molecule, also known as HSA, is composed mostly of carbohydrate with a protein core of only 31-35 amino acids in the human and 27 amino acids in the mouse. CD24 is now included in the large group of phosphatidylinositol-anchored membrane proteins. Studies with purified CD24 applied to latex beads have demonstrated a self-binding property suggesting that the molecule is capable of promoting homotypic aggregation, binding, and cell adhesion. This is supported by earlier observations of the blockade of B cell lymphoblast aggregation with an anti-CD24 monoclonal antibody. Recent studies of murine CD24 have assigned a role for CD24 as a ligand for P-selectin, a calcium-dependent lectin.