Regions containing intronic poly(A) signal sequences regulate relative abundance of Flt1. (A and D) Schematic representation of human FLT1 gene in the region of exons 13 and 14. The relative position of the deletions (A) or mutations (D) are shown. (B and E) RPA demonstrating relative abundance of processed sFlt1 and processed Flt1 in cells transfected with Flt1 minigene constructs. The full-length construct leads primarily to processed sFlt1, and deletions (B) or mutations (E) of the proximal poly(A) alone or together with distal poly(A) increases the abundance of processed Flt1. (C) Pooled quantitated data with Flt1 expressed as a ratio of sFlt1 demonstrates that deletion of both the proximal and distal pA region robustly stimulates splicing. n = 3, mean ± SEM, P < 0.005, ANOVA by ranks, *P < 0.05 compared to full length. (F) Pooled quantitated data with Flt1 expressed as a ratio of sFlt1 demonstrates that selective mutation of the proximal pA alone or with distal pA stimulates splicing. n = 3, mean ± SEM, P < 0.005, ANOVA, *P < 0.05 compared to full length.