This guidance is intended for sponsor-investigators (hereafter referred to as sponsors) developing individualized investigational antisense oligonucleotide (ASO) drug products for a severely debilitating or life-threatening genetic disease. Most often, individuals with such diseases will not have FDA-approved treatment options and their diseases will be rapidly progressing, resulting in early death and/or devastating irreversible morbidity within a short time frame. In these situations, drug development targeted at a larger number of patients with the ASO is not anticipated because of the specificity of the mechanism of action of the ASO drug product combined with the rarity of the treatment-amenable patient population.