Retroviral envelope proteins mediate receptor recognition and membrane fusion during early infection. Endogenous envelope proteins may have kept, lost or modified their original function during evolution. This endogenous envelope protein has lost its original fusogenic properties. SU mediates receptor recognition. TM anchors the envelope heterodimer to the viral membrane through one transmembrane domain. The other hydrophobic domain, called fusion peptide, mediates fusion of the viral membrane with the target cell membrane (By similarity). The surface (SU) and transmembrane (TM) proteins form a heterodimer. SU and TM are attached by noncovalent interactions or by a labile interchain disulfide bond (By similarity). The surface protein is not anchored to the membrane, but localizes to the extracellular surface through its binding to TM. Specific enzymatic cleavages in vivo yield the mature SU and TM proteins. Insertional polymorphism. Provirus present in 16% of tested individuals. Has a type 2 genome. The HERV-K(HML-2) family contains type 1 and type 2 genomes depending on the absence or presence of 292 nucleotides at the 5'-end of the env gene resulting in Env proteins of distinct sizes. Despite their overall retroviral envelope structure HERV-K(HML-2) type 1 envelope proteins lack a predictable signal sequence. Subgenomic RNA transcripts coding for full-length envelope proteins have been detected for both type of genomes. Intragenic, in first intron of DEFB107 gene. Belongs to the beta type-B retroviral envelope protein family. HERV class-II K(HML-2) env subfamily.