20XUAS regulatory sequences are fused upstream of sequence encoding GCaMP6f in inverted orientation. The GCaMP6f sequence is flanked on the 5' side by an attP site of 34bp and on the 3' side by an attB site. The attP and attB sites are oriented such that in the presence of phiC31:int, recombination between them inverts the GCaMP6f sequence, so that it is in the correct orientation to be expressed under the control of the UAS sequences.