Ly-6G/Ly-6C (Gr-1) is a 21-25 kD myeloid differentiation antigen that is a glycosylphosphatidylinositol (GPI)-anchored protein expressed on granulocytes and macrophages. In mouse bone marrow, the expression levels of Gr-1 directly correlate with granulocyte differentiation and maturation, and it is also transiently expressed on bone marrow cells in the monocyte lineage. This antigen is commonly used as a marker for identifying neutrophils and other myeloid cells, and it plays a role in antitumor immunity through its expression on immature myeloid cells.