Synaptotagmin 2 is ectopically overexpressed in excitatory presynapses of a widely used CaMKIIα-Cre mouse line

The CaMKIIα-Cre mouse line, one of the earliest established Cre driver lines, has resulted in over 800 papers to date. Here, we demonstrate that the second most widely used CaMKIIα-Cre line, Tg(Camk2a-cre)2Gsc (or CamiCre), shows ectopic overexpression of synaptotagmin 2, the most efficient Ca2+ sensor for fast synchronous neurotransmitter release, in excitatory presynapses of Cre+ brains. Moreover, RNA-seq analysis showed aberrant expression in Cre+ hippocampus, including upregulation of immediate early genes, such as Arc and Fos , and genes presumably derived from bacterial artificial chromosome transgene, such as Slc6a7 . Most importantly, CamiCre+ mice showed functional phenotypes, such as hyperactivity and enhanced associative learning, suggesting neural activities are affected. These unexpected results suggest difficulties in interpreting results from studies using the CamiCre line and raise awareness of potential pitfalls in the use of Cre driver lines in general. ### Competing Interest Statement The authors have declared no competing interest.