Fluorescent light micrograph of mice motor neuron nuclei labelled with GFP (green fluorescent protein, green). They are also counterstained with antibodies against the neuronal marker TUJ1 (red). These motor neuron nuclei are derived from embryonic stem cells that had a GFP tag inserted at motor neuron transcription factor HB9 site using CRISPR-Cas9. CRISPR-Cas9 is a gene editing technique used to cut DNA at a specific point.