a, Design of Rosa26-lsl-mGCaMP6s and Rosa26-lsl-GCaMP6s knockin transgenic mice. MARCKS: N-terminal myristoylation sequence of myristoylated alanine-rich C-kinase substrate. b, Representative confocal images showing expression of mGCaMP6s (anti-GFP) in cortical OPCs (anti-NG2) 4 weeks after tamoxifen injection. c, Quantification of mGCaMP expression by OPCs (n = 3 mice). d, Representative images showing expression of cytosolic GCaMP6s in cortical OPCs 4 weeks after tamoxifen injection. e, Quantification of GCaMP6s expression by OPCs (n = 3 mice). f-g, Representative images of single mGCaMP6s- (f) and GCaMP6s-expressing (g) OPCs. Magnified views of their distal processes (yellow arrowheads) from regions highlighted by white squares shown below. Note limited cytosolic GCaMP6s expression in the processes.