Concomitant with the completion of reverse transcription, the pre-integration complex is formed by shedding of some viral proteins from the viral core, and binding of cellular proteins, thereby yielding complexes capable of integration. The terminal cleavage reaction takes place in the cytoplasm, where two nucleotides are removed from each viral DNA 3' end. This serves to remove heterogeneous extra bases from the viral DNA ends occasionally added by reverse transcription, thereby yielding a homogeneous substrate for downstream steps, and also serves to stablilize the PIC. The DNA in PICs is considerably compacted relative to its length when fully extended, probably due to binding of proteins in addition to the viral integrase. These proteins are not fully clarified, due to the difficulty of biochemical analysis of small amounts of material, but candidates include the viral NC and MA proteins, and the cellular HMGA, BAF, and PSIP1/LEDGF/p75 proteins. Purified integrase is capable of carrying out the terminal cleavage and initial strand transfer reactions.