The HB7 antibody is specific for human CD38, a 45 kDa type II transmembrane glycoprotein expressed on thymocytes, plasma cells, and monocytes as well as other non-hematopoietic cells. T and B lymphocyte expression is discontinuous and varies based on differentiation state – present on immature cells, low on intermediate stages, and high on activated mature cells. It is found on most CD34+ cells, but not on pluripotent stem cells. CD38 is an ectoenzyme that functions to catalyze the synthesis and hydrolysis of cyclic ADP-ribose and is involved in cell signaling, regulating cell adhesion, and activation. Additionally, CD38 has been shown to be a prognostic marker for some leukemias and other diseases.