To identify the putative cellular targets of canonical BMP signaling, tissue sections collected from lungs at different developmental stages were stained with antibodies specific for appropriate cellular markers. Double immunostaining for eGFP and CD31 (PECAM-1), a surface marker of mature endothelium, or VEGFR2, a surface marker of vascular progenitor cells ( Figure  S1 ), demonstrated that the majority of CD31 pos or VEGFR2 pos cells at E12, E14.5 and E17 were also eGFP pos ( Figure 3A ), confirming that the network of eGFP pos cells in the parenchyma is composed of vascular cells and that the canonical BMP pathway is activated during the establishment of the vascular network in the developing lung. At P1, ,40% of the CD31 pos cells were eGFP pos , from P15 onwards the number of eGFP pos -CD31 pos cells declined and in the adult they were undetectable. EGFP expression with similar kinetics was observed in the endothelial cells lining the large blood vessels ( Figure 3A ). Very few eGFP pos alpha smooth muscle actin (aSMA) double positive cells were found in the walls of the vessels ( Figure 3A , white arrow in P1 vessel).