LEP and rheumatoid arthritis: Several studies have found a positive correlation between serum and synovial leptin levels and RA pathology (Otero et al., 2006; Targońska-Stępniak et al., 2008; Yoshino et al., 2011; Olama et al., 2012), but there are controversial results (Anders et al., 1999; Popa et al., 2005; Hizmetli et al., 2007; Oner et al., 2015).