Relationship between Xq22.3 HERV-W Env, MSRV Env, and Syncytin-1. An amino acid sequence alignment of Xq22.3 HERV-W Env, MSRV Env (clones AF127228 and AF331500), and Syncytin-1 is shown. The sequence of a HERV-W element on chromosome 5p12 from which the C-terminal region of the MSRV env clone AF331500 is derived (see also Table 3 and Additional file 2) is also shown. For the sake of simplicity, only the C-terminal region of the 5p12 element is included. The region of MSRV Env (AF331500) originating from HERV-W 5p12 is highlighted in yellow. Predicted signal peptides (according to SignalP 3.0, http://www.cbs.dtu.dk/services/SignalP/) are shaded in gray. The stop codon at position 39 of Xq22.3 HERV-W Env and AF127228 is indicated by a dot (·). The consensus C-X-X-C motif conserved among C-type and D-type retroviral Env proteins  is shown in boldface. The border between the SU and TM regions is indicated by arrows. The proteolytic cleavage site (consensus R/K-X-R/K-R) between SU and TM is highlighted in red letters. The sequences of the MSRV Env SU protein (generated using the MSRV env clone AF331500) studied by Rolland et al.  is marked in red. The fragment of the MSRV env clone AF127228 used for generation of the anti HERV-W Env monoclonal antibody 6A2B2  is shown in green.