CD28 and systemic lupus erythematosus: In the previous study, we investigated the association between systemic lupus erythematosus (SLE) and the SNPs of the genes that were involved in regulating T-cell activation, including cytotoxic T-lymphocyte-associated protein 4 (CTLA4), CD28, programmed cell death protein 1 (PDCD1), and inducible T cell costimulator (ICOS) (4).