Calcium (Ca2+) can be mobilised from intracellular stores by the presence of nicotinic acid adenine dinucleotide phosphate (NAADP). Two pore calcium channel proteins 1 and 2 (TPCN1 and 2) are expressed on endosomal (not shown here) and lysosomal membranes and mediate the mobilization of Ca2+ from these organelles when activated by NAADP (Brailoiu et al. 2009, Calcraft et al. 2009).