Surface expression of CCR2, CCR5, CXCR4, and CD71 in monocytes at different stages of differentiation. Monocytes (day 0, 1, and 7) were directly or indirectly stained with specific antibodies and analyzed by FACS. The immunofluorescence profile obtained for each antibody was compared with that of its corresponding control and the result is shown as an open curve. The level of expression of each surface antigen is calculated by differences between the level of staining with a specific antibody and the baseline of the control antibody. Similar profiles were obtained with 3 different donors.