Cyclophilin B (CyPB) is a member of the cyclophilin family with specific N- and C-terminal extensions. CyPB is a heparin-binding protein first identified as a receptor for cyclosporin A(CsA). CsA blocks helper T-cell activation at a step between T-cell receptor stimulation and the transcriptional activation of cytokine genes. Cyclophilins from many species possess peptidyl-prolyl cis-trans isomerase (PPIase) activity that is blocked by CsA and therefore may be relevant in CsA-mediated immunosuppression. Unlike CyPA, CyPB has a signal sequence leading to its translocation in the endoplasmic reticulum. CyPB is secreted in biological fluids such as blood or milk and binds to a specific receptor present on the human lymphoblastic cell line Jurkat and on human peripheral blood lymphocytes.