Neurons from a mouse brain stained with GFP (green fluorescent protein) [Source]
I think I’ve shown quite a few images with GFP in the past, but I’m not sure I’ve ever talked about it. GFP is a protein that was isolated from jellyfish (Aequorea victoria) and fluoresces green. The DNA for GFP has been isolated and can be inserted into the genome at certain types of markers, so it will only be expressed in specific types of cells. Here it is in neurons, but it can be expressed in just GABAergic neurons, or just glia, or any type of cell really, depending on where you put it in the code, and what type of promoter it will follow (how specific it is).