Intracisternal A-particle Gag-related polyprotein: Intracisternal A particles (IAPs) are defective retroviral elements. Due to extensive mutations in the envelope coding sequence, IAPs can only form defective viral particles confined to the intracisternae of the Golgi. IAPs are an important class of transposable elements that induce genomic mutations and cell transformation by disrupting gene expression. Readthrough of the terminator UAA occurs between the codons for Met-218 and Ala-219.