Accordingly, in IBM we found a highly abundant neighborhood group (group 6) within the COL15A1-expressing FAP population that was characterized by expression of genes associated with fibrosis, tissue remodeling (for example TNC, POSTN and LOXL2)25–29 and inflammation (for example, CXCL10) representing an IBM-specific cell state of endomysial FAPs (Fig. 3c–e, Extended Data Fig. 5c,f,g and Supplementary Table 13). Here, COL15A1 is linked to inclusion body myositis.