20XUAS regulatory sequences are separated from sequence encoding GCaMP6f by a "SPARC-I" module that comprises: an attP site of 38bp, a STOP cassette, a second attP site (60bp, full-length site), a spacer and an attB site. phiC31:int catalyzes irreversible recombination between the attB site and one of the two attP sites; if the attP site upstream of the STOP cassette is utilized, the STOP cassette is excised, permitting expression of GCaMP6f, while if the attP site downstream of the STOP cassette is utilized, only the spacer is removed, and the STOP cassette is preserved. The "SPARC-I" module typically results in expression of the effector (in this case GCaMP6f) in approximately 15% of cells (recombination of the attB site with the full-length attP downstream of the STOP cassette is more likely than recombination with the shortened attP site upstream of the STOP cassette).