Given the ability of Axl and MerTK ectodomains to be cleaved by metalloproteases11, and in keeping with previous reports showing that soluble Axl (sAxl) is abundant in the synovial fluid of RA patients6, we confirmed that both Axl-bearing and Axl-negative cells expressed the cleaving enzyme ADAM10, including within the synovial region facing the joint cavity (Fig. 3E). Here, MERTK is linked to rheumatoid arthritis.