Steps taken to isolate cardiac FAPs are shown in Online Figure I. In brief, non-cardiac myocyte fraction of heart cells was sorted to isolate cells that express PDGFRA and exclude those expressing hematopoietic lineage markers CD32, CD11B, CD45, Lys76, Ly−6c and Ly6c (Linneg), stem cell and fibroblast marker THY1, and fibroblast marker DDR2. The isolated mouse PDGFRApos:Linneg:THY1neg:DDR2neg cells were also analyzed for the expression of lineage markers TIE2, PDGFRB, CD146, and KIT antigen by flow cytometry and/or immunostaining.