The targeting vector was designed with (from 5' to 3') a CMV-IE enhancer/chicken beta-actin/rabbit beta-globin hybrid promoter (from pCAGGS), a loxP-flanked STOP cassette (stop codons in all three reading frames linked to synthetic pA-hGHpA-PGKpA), the GCaMP6f coding sequence, a WPRE sequence (to enhance mRNA transcript stability), a bovine growth hormone polyA signal, the GCaMP6f coding sequence (described below), a woodchuck hepatitis virus post-transcriptional regulatory element (WPRE; to enhance mRNA transcript stability), a bovine growth hormone polyA signal, an AttB site, a PGK promoter-Neomycin resistance gene-PGK polyA cassette, and an AttP site. This entire targeting vector was inserted between exons 1 and 2 of the Gt(ROSA)26Sor locus. GCaMP6f has a chicken smooth muscle M13 fragment of myosin light chain kinase, a circularly permutated EGFP, and a rat calmodulin DNA fragment.