However, in practice a number of factors may limit the ability of genetically-modified cells to expand and restore immune function in vivo, including the ongoing production of unmodified CD4+ T cells susceptible to HIV-1 infection, limitations in the proliferative capacity of modified cells, indirect mechanisms of cell death, and processes such as lymph node fibrosis that may affect the efficiency of CD4+ T cell reconstitution [29]. Here, CD4 is linked to HIV-1 infection.