We conducted single-cell RNA sequencing analysis of mitral valve leaflets extracted from six patients, comprising three individuals with moderate-to-severe functional mitral regurgitation and three nondiseased mitral valve controls. Bioinformatics was used to identify the cell types, describe the cell functions, and investigate cellular developmental trajectories and interactions. Overall design: We conducted single-cell RNA sequencing analysis of mitral valve leaflets extracted from six patients, comprising three individuals with moderate-to-severe functional mitral regurgitation and three nondiseased mitral valve controls. Bioinformatics was used to identify the cell types, describe the cell functions, and investigate cellular developmental trajectories and interactions. ------------------------ Authors state: Raw sequence data were provided at China Genomic Sequence Archive (GSA). The accession number will be included once the paper is accepted