As for activation by ligands, all groups agree that TPCs are modulated by PI(3,5)P2, though its role is considered by some to be more of a 'permissive' factor rather than an acute signal per se. As for NAADP itself, the conclusion in 2012 that TPCs are not involved in NAADP signalling was partly due to the fact that their transgenic mice (designed to knockout both TPC1 and TPC2; double-knockout, DKO) apparently retained sensitivity to NAADP. However, others have questioned whether these mice are true DKO when they are predicted to retain >90% of the TPC protein sequences (i.e. they express only mildly truncated TPCs which are functional ). In a different DKO mouse that is demonstrably TPC-null, NAADP responses are completely abolished.