Enhancers often drive gene expression in too many cells to be experimentally useful. To restrict gene expression to a smaller number of cells, one can “filter” expression using a flip-out cassette construct as shown in the example below.

This method was described in Awasaki et al. (2014): "Making Drosophila lineage-restricted drivers via patterned recombination in neuroblasts", Nature Neuroscience 4: 631-637.

Stocks useful for filtering gene expression through multiple rounds of recombinase-mediated cassette removal are shown below.