Since i21VEGFR-1, transcript and protein is the main VEGFR-1 isoform expressed in MDA-MB-231 breast cancer cells, our initial approach to study the function of this isoform consisted in inhibiting its expression by RNA interference or overexpressing the intracellular isoform by transfection of i21VEGFR-1 [23]. The gene discussed is FLT1; the disease is breast carcinoma.