Flt-1 isoform expression during a developmental time course. WT ES cell cultures were used (day 0) or differentiated for the indicated number of days, and total RNA was isolated for real-time PCR analysis. (A) The relative copy number of sFlt-1 or mFlt-1 RNAs were calculated relative to 18S RNA. (B) The ratio of sFlt-1/mFlt-1 RNA over time. Error bars represent SEM.