Significant RD-associated reductions in periostin concentrations were observed in studies of patients with ankylosing spondylitis (SMD = −0.70, 95% CI −1.26 to −0.15, p = 0.013; I2 = 75.1%, p = 0.045) and dermatomyositis (SMD = −0.49, 95% CI -0.91 to -0.07, p = 0.022; I2 = 22.6%, p = 0.26), with a lower heterogeneity observed in the latter subgroup (Fig. 6). The gene discussed is POSTN; the disease is ankylosing spondylitis.