Somewhere over the brainbow

First, researchers created a gene which could be recombined in various arrangements to express one of four different colours. Then, multiple copies of this gene were inserted in the genome of mice or other species, resulting in the random expression of various colour ratios.

Due to the random nature in the expression of these genes, researchers are currently unable to precisely control the labeling process, which makes it very hard to identify specific neurons. They get a good idea of what the circuit looks like, but not exactly which class of cells each neuron belongs to.