CD34（小鼠、大鼠的同源蛋白寫作Cd34），是一類分化簇（Cluster of differentiation, CD）分子，在人體內由CD34基因編碼，主要用來鑑定造血幹細胞（HSC）等成體幹細胞[5][6][7]。CD34化學本質爲糖蛋白，正常情況下位於細胞膜表面，功能包括參與細胞—細胞間黏附和細胞—細胞外基質（ECM）黏附。
CD34最早於20世紀80年代由兩個課題組同時獨立發現，最初是用來鑑定一類造血幹細胞（CD34+陽性）。现在，是否表達CD34也是分離、鑑別造血幹細胞及其亚群（subpopulation）的一項重要指標[8][9][10][11][12][13]。CD34为单跨膜的黏液素（Mucin）類糖蛋白，通常分佈於細胞膜表面[14]。CD34功能複雜，至今CD34的功能仍然未完全闡明，已知的CD34分子功能主要是參與細胞黏附[15][16]。
CD34蛋白主要表達於造血幹細胞和造血祖細胞中，同時，血管內皮細胞、一部分間充質幹細胞（MSC）也表達CD34[17]。CD34在臍帶和骨髓中表達強度相對較高[18]。
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Furness SG, McNagny K. Beyond mere markers: functions for CD34 family of sialomucins in hematopoiesis. Immunologic Research. 2006, 34 (1): 13–32. PMID 16720896. doi:10.1385/IR:34:1:13. Drew E, Merzaban JS, Seo W, Ziltener HJ, McNagny KM. CD34 and CD43 inhibit mast cell adhesion and are required for optimal mast cell reconstitution. Immunity. Jan 2005, 22 (1): 43–57. PMID 15664158. doi:10.1016/j.immuni.2004.11.014. Strilić B, Kucera T, Eglinger J, Hughes MR, McNagny KM, Tsukita S, Dejana E, Ferrara N, Lammert E. The molecular basis of vascular lumen formation in the developing mouse aorta. Developmental Cell. Oct 2009, 17 (4): 505–15. PMID 19853564. doi:10.1016/j.devcel.2009.08.011.
