Furthermore, even though FAPs expansion and regenerative fibrogenesis have a positive impact on acute muscle regeneration (Heredia et al., 2013; Fiore et al., 2016), the evidence that IL-15 expression is positively correlated with the number of FAPs and collagen deposition in subjects with rotator cuff tear indicates that IL-15 might serve, instead, as a signal to sustain FAPs pathogenic fibrogenesis in chronically fibrotic muscles. This evidence concerns the gene IL15 and rotator cuff syndrome.