Although the description of our characterization of the Col1a1FRT-Cre-ER-T2-FRT allele remains valid, additional characterization of FlpO-activated sarcoma cell lines and mouse embryo fibroblasts (MEFs) from the Col1a1FRT-STOP-FRT-Cre-ER-T2 mice show inefficient recombination of FLOX alleles when exposed to 4-OHT in vitro. Here, FLT1 is linked to sarcoma.