17Many experimental approaches rely on controlling gene expression in select subsets of 18 cells within an individual animal. However, reproducibly targeting transgene expression 19 to specific fractions of a genetically-defined cell-type is challenging. We developed 20Sparse Predictive Activity through Recombinase Competition (SPARC), a generalizable 21 toolkit that can express any effector in precise proportions of post-mitotic cells in 22Drosophila. Using this approach, we demonstrate targeted expression of many 23 S2) 7 . The core of this toolkit is a set of bistable UAS-driven constructs that can be 46 switched on or off in different relative proportions of cells, depending on their 47 expression of each SPARC construct in one of the largest genetically-defined 62 populations of neurons in the Drosophila optic lobe, T4 and T5 cells 11 , in animals that 63 express PhiC31 pan-neuronally. Across SPARC-GCaMP6f variants, we observed 64 progressively fewer labeled neurons (Fig. 1C-E). SPARC-attP60-GCaMP6f labeled 65 many overlapping neurons, SPARC-attP38-GCaMP6f labeled an intermediate number 66of neurons and SPARC-attP34-GCaMP6f labeled individual neurons whose dendrites 67 could be visualized (Fig 1E inset). We therefore named these variants SPARC-D individual T5 cells relied on laborious FlpOut approaches that required titrated and 108 temporally precise heat shocks of Drosophila larvae to restrict effector expression to a 109 subset of cells [16][17][18] . In contrast, the SPARC method consistently labeled fewer T5 110 neurons, and labeled them more sparsely, than the sparsest FlpOut labeling using brief 111 and developmentally late heat-shock ( Fig. 1E, 2A,B). More importantly, when we 112 imaged visually-evoked Ca 2+ responses in regions of interest (ROIs) representing T5 113 dendrites, we observed that the fluorescent signals from SPARC-labeled ROIs were depolarized ( Figure 2I, J). Indeed, tdTomato -R2 neurons were slightly hyperpolarized 130 by light ( Figure 2I, J), implying that these R2 neurons were postsynaptic to other R2 131 neurons that express CsChrimson. Thus, the SPARC system enables optogenetic 132 activation of sparse cell populations within a single cell type, allowing us to discover 133 evidence for mutual inhibitory interactions within a cell type. Although it was already 134 known that some R neuron types inhibit other R types 20 , it was not previously known 135 that there is mutual inhibition between R neurons of the same type.