If we wanted to add genetically modified cells to a person's (or mouse) brain we would take skin cells (fibroblasts) from the person and turn those cells into pluripotent stem cells, aka Induced Pluripotent Stem Cells (IPS cells). IPS cells have the potential to turn into any cell type, including neurons, and because they are your own cells then they would not cause an immune reaction and potential rejection. The IPS cells would be grown in a dish and the genetic modification would be performed using the CRISPR/Cas9 genome editing system. If we wanted the modified IPS cells to turn into neurons we would push them to go from stem cells to neurons for a few days in the dish. After a few days the cells would start to look like neurons and then those neuron-like cells would be put in a needle to inject it into the area of the brain that we want the new cells to be located.

This type of experiment has been performed with mice and it has been shown that human IPS neurons can grow and become a part of the mouse brain!