 Geneticists have created genetic varieties of model organisms (typically fruit flies), called "GAL4 lines", each of which expresses GAL4 in some subset of the animal's tissues.For example, some lines might express GAL4 only in muscle cells, or only in nerves, or only in the antennae, and so on.For fruit flies in particular, there are tens of thousands of such lines, with the most useful expressing GAL4 in only a very specific subset of the animal—perhaps, for example, only those neurons that connect two specific compartments of the fly's brain.