In 2005, we began our efforts to generate GAL4 lines that would have sparser expression patterns than the widely-used enhancer trap lines. We used 2 to 3 kb fragments of genomic DNA to drive expression of GAL4 from constructs inserted into the same genomic site (see Pfeiffer et al. 2008). We went on to generate over 7,000 of these so-called Generation 1 driver lines, and in collaboration with the FlyLight project team, imaged their expression patterns in the brain and VNC of the adult (see Jenett et al. 2012).
These lines have been widely used and have been distributed to over 500 laboratories. However, they do not produce cell-type-specific expression in the adult brain. To achieve this level of specificity, in 2011 we began to generate lines whose expression was limited to the positive intersection of the expression patterns of two enhancers. These split-GAL4 lines can indeed produce the desired specificity.