Mapping the contribution of the third intracellular loop of HuPAR2 for PERV-A infection. The third intracellular loop (ICL3) contains eighteen single amino acid differences, between HuPAR1 and HuPAR2, and a three amino acid insertion in HuPAR2. In order to determine what sequences in HuPAR2 ICL3 are required in addition to the third extracellular loop (ECL3) of HuPAR2, two HuPAR2 regions, I and II, were swapped. (A) shows the location of Regions I and II and the respective sequence differences (in bold) between HuPAR1 and HuPAR2. (B) shows chimera function for PERV-A 14/220* infection. Region II in the HuPAR1(HuPAR2 ECL3)eGFP backbone results in a statistically significant (p ≤ 0.03) increase, though neither Region confers full HuPAR2 function.