Thus, repetitive, noninvasive PET/CT imaging with [18F]FAHA may facilitate future preclinical or clinical studies to elucidate the roles of class IIa HDAC enzymes in tumor progression, chemoresistance, and the expansion of CSCs, and may help to optimize therapeutic doses of novel class IIa HDACIs for combined chemoradiotherapy. The gene discussed is HDAC9; the disease is neoplasm.