CD4 and systemic lupus erythematosus: Interestingly a novel splice isoform, Pbx1-d, of the PBX1 gene (regulatory locus Sle1a.1-associated gene) has been implicated in the production of activated and auto-reactive CD4 positive T cells in a mouse model of lupus, through a defective response of CD4(+) T cells to retinoic acid-directed expansion of TGFβ-induced regulatory T cells [34,66].