 Gal4 expression can be made even more specific by means of "intersectional strategies".These can combine two different GAL4 lines—say, A and B—in a way that GAL4 is only expressed in the cells that are in line A but not line B, or those that are in both lines A and B.When combined with intrinsically sparse GAL4 lines, this offers very specific selection, often limited to a single cell type.