p24 forms inner protein layer of the nucleocapsid. ELISA tests for p24 is the most commonly used method to demonstrate virus replication both in vivo and in vitro.

1: p24 antigen capture assay for quantification of human immunodeficiency virus using readily available inexpensive reagents. Wehrly K, Chesebro B; Methods 1997;12:288-293. PMID:9245608