123 PROJECT DESCRIPTION 45 Rotator cuff (RC) tears are an extremely common cause of shoulder pain and disability. Up to 50% of patients greater than the age of 65 years of age have evidence 6789 of a RC tear. In addition, patients with small asymptomatic cuff tears tend to progress to larger, symptomatic tears. The outcomes of surgical repair of small RC tears are good, but there has been limited success in the surgical treatment of massive RC tears. 11 01 Massive RC tears have been linked with fatty infiltration of the rotator cuff muscles. Patients with large RC tears who develop fatty infiltration have poorer clinical outcomes 11 23 and higher rates of failure after attempted repair. We have previously found that a newly discovered stem cell population-fibro/adipocyte progenitor (FAP) cells-is critical in the 11 45 development of fatty infiltration after RC injury. Further, these cells share similar expression patterns with the beige adipocyte lineage, a distinct cell population that has 11 67 unique thermogenic and metabolic capabilities that could improve muscle function. 11 89 The study will leverage our well-studied animal model and a novel repair model to evaluate the mechanism by which FAP cells can improve muscle function through 22 01 differentiation into a beige fat phenotype in mouse and human tissue. Understanding the relationship between FAP cells and beige adipocytes and utilizing these cells in endogenous and exogenous treatment strategies could help improve muscle quality in RC repair as well as other muscle injury states.