CD6 and Parkinson disease: However, three cytokines (Natural killer cell receptor 2B4, T‐cell surface glycoprotein CD6 isoform and Leukaemia inhibitory factor receptor) for MS and one cytokine (Fibroblast growth factor 21) for PD, the IVW and MR Egger regression methods indicated a statistically significant heterogeneity, with a p‐values <0.05 (Table 2).