ARPP21 and bacterial infectious disease: Studies have shown that the Rcs system in Salmonella can positively (low RcsB-P levels) or negatively (high RcsB-P levels) regulate the expression of the SPI-1 and SPI-2 pathogenicity island genes (Wang et al., 2007, 2009), which are important in the early stages of bacterial infection and can enable S. enterica to cross the epithelial barrier and enter macrophages (Lostroh and Lee, 2001; Fass and Groisman, 2009).