SLC15A4 and systemic lupus erythematosus: Being a proton-dependent peptide/histidine transporter, SLC15A4 protein controls the transport of various molecules from the inside of endosomes to the cytosol and has been associated inter alia with systemic lupus erythematosus (Wang et al. 2013; Lee et al. 2014; Zuo et al. 2014; Zhang et al. 2016).