CdiI immunity proteins function as part of the bacterial contact-dependent growth inhibition (CDI) system. CDI is mediated by the CdiB-CdiA two-partner secretion system. Each CdiA protein exhibits a distinct growth inhibition activity, which resides in the polymorphic C-terminal region (CdiA-CT). Cells with the CDI sytem also express a CdiI immunity protein that blocks the activity of cognate CdiA-CT, thereby protecting the cell from autoinhibition. In many CDI systems the cdiBAI genes are followed by orphan cdiA-CT/cdiI modules, suggesting that these modules are exchanged between the CDI systems of different bacteria.