PD-1 (also known as Programmed Death 1, CD279, and PDCD1) is a negative regulatory T cell surface receptor. PD-1 is a Type I transmembrane protein expressed on the plasma membrane of T-cells. It is a T-cell checkpoint protein, involved in preventing autoimmunity. Because binding of PD-1 by its ligands PD-L1 or PD-L2 has a suppressive effect on the immune system, inhibition of the PD-1/PD-L1 pathway is a major strategy of immune-oncology therapies. PD-L1 is upregulated in certain cancers, and blocking PD-1 or PD-L1 can inhibit cancer growth. Primary antibodies are available purified, or with a selection of fluorescent CF® Dyes and other labels. CF® Dyes offer exceptional brightness and photostability. Note: Conjugates of blue fluorescent dyes like CF®405S and CF®405M are not recommended for detecting low abundance targets, because blue dyes have lower fluorescence and can give higher non-specific background than other dye colors.