UAS regulatory sequences drive expression of a ratiometric Ca[2+] indicator that can be used to determine Ca[2+] levels at presynaptic boutons. The sensor is composed of the vesicular synaptic protein Syt1 fused at its C-terminal end to mScarlet (a Ca[2+]-insensitive red fluorescent protein) followed by the jGCaMP8f Ca[2+] indicator.