The HIV p24 protein is also known as the capsid protein (or CA) because it forms the genome-enclosing capsid of the HIV virion. It is processed from the gag gene to become a 24-kDa, two-domain protein. It has a flexible linker that allows many configuration options in the assembly of the capsid shell. An estimated 2000 individual p24 proteins make up the capsid, through hexamer and pentamer complexes. p24 has been used as an antigen in detection of HIV in the blood.