point mutations on five of these residues found that four of the five mutations tested abolished both CdtB's ability to degrade DNA in cell-free extracts and to cause G2/M arrest upon microinjection. The fifth mutation moderately reduced CdtB's activity. CdtA and CdtC CdtA and CdtC make up the B subunit of the CDT holotoxin responsible for targeting the CdtB against susceptible cells. Neither subunit appears highly conserved, with sequence identities between different species often lower than 30%. The molecular weight of CdtA ranges from 23 kDa to 30 kDa, whereas CdtC ranges from 19 kDa to 21 kDa depending on the species. Activity CdtA and CdtC are both believed to bind to the surface of target cells. The exact mechanism of this binding is unclear, and may not be conserved between CDT toxins from different species. Proposed targets of CdtA and CdtC binding have included cholesterol, N-linked glycans, and glycosphingolipids. Current research has produced conflicting results on the actual importance of these proposed targets. Both CdtA and CdtC contain lectin domains, suggesting that the toxin may bind via carbohydrates on the target cell's surface, whereas other research has suggested that the targets are surface proteins. Notes Toxins