library_construction_protocol: Mitral valves were isolated from B.g7 and K/B.g7 hearts and digested using Collagenase 2 and DNAse. Following digestion, cells were washed and filtered, then red blood cells lysed using AKC buffer. Cells were stained with a viability dye and live, single cells were sorted using fluorescence-activated cell sorting (FACS). Library was performed according to the manufacter's instructions (single cell 3' protocol, v2, 10x Genomics).