CD4 and HIV infectious disease: It became possible to quickly assess the status of CD4+ T cells in HIV infection (17), accurately define the stages and phenotypes of B cell development in human BM and how mutations in genes such as BTK, BLNK, IGHM, IGLL1, CD79A, CD79B, PIK3R1, and TCF3 differentially affect this process (3, 19, 20), and delineate distinct types of SCID due to different gene mutations according to the presence and absence of specific lymphocyte populations, such as B+T−NK– SCID (IL2RG, JAK3), B−T−NK+ SCID (RAG1, RAG2), B+T−NK+ SCID (IL7RA), or B−T−NK− SCID (ADA) (20, 21).