a 2.5 kb vec promoter sequence was cloned between insulators from the chicken -globin gene 29 and used to drive cre expression . r26r - lacz/+;vec - cre or r26r - yfp/+;vec - cre conceptuses were generated by crossing vec - cre/+ males with r26r - lacz/+ or r26r - yfp/+ females . runx1;vec - cre and runx1;vec - cre conceptuses were generated by crossing runx1 ; vec - cre/+ males with runx1 females . runx1;r26r - yfp/+;vec - cre or runx1;r26r - yfp/+;vec - cre conceptuses were generated from crossing runx1;r26r - yfp/+ ; vec - cre/+ males with runx1 females . conceptuses were suspended in phosphate - buffered saline or in a 1:2 mixture of benzyl alcohol / benzyl benzoate and visualized with a stereomicroscope . x - gal ( sigma ) staining was performed as described previously 4 . in some cases the conceptuses were also incubated with rat anti - mouse pecam1 and abc reagent at 4c 30 , then treated with dab peroxidase substrate . conceptuses were embedded in paraffin , sectioned , and counterstained with nuclear fast red . to analyze intra - aortic clusters , embryos were fixed and stained with anti - c - kit and anti - pecam1-antibodies . a 1:2 mix of benzyl alcohol / benzyl benzoate was used to increase the transparency of tissues . three - dimensional reconstructions were generated from z - stacks ( 6287 serial sections ) . methylcellulose colony forming assays were performed as described previously 4 and colonies counted after 7 days . immuno - staining was performed with phycoerytherin , allophycocyanin , or alexa fluor 647 conjugated antibodies stained cells were analyzed on a becton dickinson facscalibur flow cytometer . a 2.5 kb vec promoter sequence was cloned between insulators from the chicken -globin gene 29 and used to drive cre expression . r26r - lacz/+;vec - cre or r26r - yfp/+;vec - cre conceptuses were generated by crossing vec - cre/+ males with r26r - lacz/+ or r26r - yfp/+ females . runx1;vec - cre and runx1;vec - cre conceptuses were generated by crossing runx1 ; vec - cre/+ males with runx1 females . runx1;r26r - yfp/+;vec - cre or runx1;r26r - yfp/+;vec - cre conceptuses were generated from crossing runx1;r26r - yfp/+ ; vec - cre/+ males with runx1 females . conceptuses were suspended in phosphate - buffered saline or in a 1:2 mixture of benzyl alcohol / benzyl benzoate and visualized with a stereomicroscope . x - gal ( sigma ) staining was performed as described previously 4 . in some cases the conceptuses were also incubated with rat anti - mouse pecam1 and abc reagent at 4c 30 , then treated with dab peroxidase substrate . conceptuses were embedded in paraffin , sectioned , and counterstained with nuclear fast red . to analyze intra - aortic clusters , embryos were fixed and stained with anti - c - kit and anti - pecam1-antibodies . a 1:2 mix of benzyl alcohol / benzyl benzoate was used to increase the transparency of tissues . three - dimensional reconstructions were generated from z - stacks ( 6287 serial sections ) . methylcellulose colony forming assays were performed as described previously 4 and colonies counted after 7 days . immuno - staining was performed with phycoerytherin , allophycocyanin , or alexa fluor 647 conjugated antibodies stained cells were analyzed on a becton dickinson facscalibur flow cytometer .