HERV-K(HML-2) expressed at RNA and protein level in ES/iPS cells. A) Prototypic HERV-K(HML-2) open reading frames B) Type 1 and 2 mRNA species, ∆ denotes the 292 bp deletion in type 1 proviruses. 1) full length type 2, 1∆) full length type 1, 2) env mRNA type 2, 2∆) env mRNA type 1, 3) 1.8 kb mRNA type 2 encoding rec, 3∆) 1.5 kb mRNA type 1 encoding Np9, 4) 1.5 kb mRNA derived from type 1 or type 2 . PCR primers are indicated by arrows and arrowheads. Primers PrevENV, PrevREC, PrevNP9 overlap the immediate upstream splice site. C) RT-PCR of HERV-K(HML-2) expression in ES cells (H1, H9, BG01), fibroblasts (MRC5, MSC, hFib2) and corresponding iPS cells using the primer pair K20 and K6. OCT4, NANOG served as pluripotency, LMNA as differentiation markers, ACTB as internal control [19, 20]. D) Western blot for HERV-K(HML-2) Gag, OCT4 and ACTB in ES, iPS, fibroblast and EC cells.