Intracisternal A-type particles (IAP) are defective endogenous retroviruses that accumulate in the endoplasmic reticulum (ER) of rodent cells. The enveloped particles are produced by assembly and budding of IAP Gag polyproteins at the ER membrane. Readthrough of the terminator UAA occurs between the codons for Met-218 and Ala-219. This particle is a defective retrovirus.