Specificity of HERV-E.CD5 transcripts for B-1 lymphocytes. (a) Tonsillar B-1a, B-1b, and B-2 cells were sorted, based on their membrane cell surface expression of CD5 and CD45RA. (b) The numbers of HERV-E.CD5 (E1B) and conventional CD5 transcripts (E1A) were determined by RT-PCR using, respectively, CD5-E1B plus CD5 E5-6, and CD5 E1A plus CD5 E5-6 primers. GAPDH house keeping gene was also used. (c) Detection of HERV-E fusion transcripts and HERV-E transcripts (HERV-E 4-1; HERV-E 7p 22.1) in different cells, and various human tissues. Amplification used primers with a sens primer located within the 5′LTR just upstream the splice donor and a reverse primer (see details in Table 2).