The HIV test has now evolved to the fourth generation, so why does the fourth generation reagent shorten the window period? That is determined by the genetics of HIV. The fourth generation reagents can detect P24 antigen, which is encoded by the gag gene, and its encoded core protein contains P24 and P17. The gag gene is the most conserved in the HIV genome and can stimulate the body to produce antibodies, and antibodies against the gag protein are the earliest antibodies to be measured after infection. Therefore, the current immunoblot strips must contain antibodies encoded by gag. Then, by the time the antibodies are detected in the body, the antigens have been present in the body for some time, so the antigens are detected before the antibodies. p24 antigens can shorten the window period to 2 weeks.