LEP and systemic lupus erythematosus: Inclusion criteria: cohort, case-control, and cross-sectional studies with data on the serum/plasma leptin levels, leptin receptor levels, and relevant gene polymorphism in case and control groups, studies with predefined SLE criteria and measurement methods, studies with OR estimates with 95% CI and p-values. Exclusion criteria: case reports, editorials, letters, comments, review articles, meta-analysis, and non-human subjects