CD53 is a pan-leukocyte surface glycoprotein which spans the plasma membrane four times and is a member of the transmembrane 4 superfamily (TM4SF). CD53 is highly conserved in evolution, mouse CD53 was 91% identical to rat and 82% identical to human CD53. Mouse CD53 has a molecular mass of 35-45 kD and expresses on virtually all peripheral leukocytes, immature CD4-CD8- double negative thymocytes, and functionally mature CD4+CD8- or CD4-CD8+ single positive subsets. CD53 is proposed to play an important role in thymopoiesis and leukocyte signal transduction. It was reported that CD53 interact with a number of proteins, including CD2, CD9, CD19, and CD21. Cross-linking of CD53 promotes B cell activation. It was found that there is a strong correlation between CD53 expression and positive selection of thymocytes.