 On the butterfly body a hydrophobic part fills a small pocket which is mainly formed by the side chains of Lys-103, Val-106 and Val-179.However many other NNRTIs have been found to bind to RT in different modes.Second generation NNRTIs such as diarylpyrimidins (DAPYs), have a horseshoe-like shape with two lateral hydrophobic wings and a pyrimidine ring which is the central polar part.