FAP and breast adenocarcinoma: Additionally, a previous study demonstrated that FAP+ tumor-associated macrophages (TAMs) reside in human mammary adenocarcinoma [18], and Muliaditan et al. recently discovered that a collagen I-rich wound-like microenvironment helps maintain the FAP+ TAM phenotype in 4T1 mammary adenocarcinoma cell lines [33], which indicates that as the substrate of FAP, collagen I may also participate in educating infiltrated immune cells with upregulated FAP expression to promote tumor growth and invasion.