To investigate the circulating CD4+ Treg compartment of SLE patients, we performed a detailed flow cytometric characterization of cryopreserved PBMCs from two cohorts of patients: (i) a clinic-attending—cohort 1- of patients recruited at the time of their regular clinic visit; and (ii) a population-based cohort—cohort 2—of patients recruited outside of their regular clinic visits through the Cambridge BioResource. This evidence concerns the gene CD4 and systemic lupus erythematosus.