Predisposing factors of reduced BMD in this population have been identified, including some in common with the non-HIV infected population (age, male sex, a lower BMI, active tobacco use and methadone therapy) as well as some specific factors related with HIV-infection (low CD4 cell count, HCV co-infection, longer duration of HIV infection and of combination antiretroviral treatment, specially the duration of PI/r). The gene discussed is CD4; the disease is HIV infectious disease.