Induction of calpain1 expression by syncytin-1 knockdown. (A) Western blot analysis. At various post-transfection time points BeWo cells were examined for calpain1 expression levels. At 48, 60, 72 and 84 hours post-transfection, increased calpain1 protein levels were detected in the cells transfected with syncytin-1-siRNA (SsiRNA1, marked as K) compared to cells transfected with control siRNA (marked as C). β-actin was detected in the same blot as protein loading controls. (B) Calpain1 mRNA levels were determined by real-time PCR. At the 48, 60, 72 and 84 hours post-transfection, significantly increased calpain1 mRNA levels were found in the syncytin-1 knockdown (SsiRNA1) groups compared to the correspondent control (Ctl siRNA) groups (**, P < 0.01; *, P < 0.05).