Despite the progress in medicine, no significant advancement in the standard of care for glioblastoma (GBM) patients have been reached. GBM heterogeneity, poor blood–brain barrier penetration and resistance to therapy highlight the need for new targets and clinical treatments. A step toward clinical translation includes the eradication of GBM Tumor-Initiating Cells (TICs), responsible for GBM heterogeneity and relapse. By using patient-derived TICs and xenograft orthotopic models, we demonstrate that Lysine-specific histone demethylase 1A (LSD1) is a druggable target in GBM. Here, we analyze the effect of LSD1i treatment on histone H3K4 in primary human cells and mouse brains.