Fig. 1. Diagrams of HERV-K proviruses and their transcripts.

A compete sequence of HERVs are composed of gag, pol, pro, and env regions sandwiched between two long terminal repeats (LTRs). Gag encodes the structural components of matrix (MA), capsid (CA), and nucleocapsid (NC). The products of pol gene are reverse transcriptase (RT), integrase (IN), and RNase H (RH). The pro mainly encodes the enzyme protease (PR), while env encodes Env surface (SU) and transmembrane (TM) subunits. LTRs are composed of U5 region, U3 region and repeat sequences (R). The HERV-K (HML-2) usually expresses a full-length transcript (8.6-kb) and encodes the gag, pro, and pol polyproteins. Env gene transcripts two singly spliced products, a 3.3-kb product to encode Env polyprotein, and a 1.5-kb product of unknown function known as the hel transcript, and a doubly spliced product (1.8-kb) to encode either the Rec or Np9 accessory proteins depending on the presence or absence of a 292-bp deletion at the pol/env boundary