To identify the cell population vulnerable to NMDAR toxicity, ALS patient-derived organoids were immunostained with antibodies to the nuclear proteins, NeuN and myelin transcription factor 1-like (Myt1L) protein, which identify post-mitotic neurons, and with antibodies to the astrocyte marker, glial fibrillary acidic protein (GFAP). Here, GFAP is linked to amyotrophic lateral sclerosis.