The purpose of this guidance is to describe the nonclinical information that FDA recommends to support an investigational new drug application (IND) for an antisense oligonucleotide being developed to treat a severely debilitating or life-threatening (SDLT) disease caused by a unique genetic variant where only a small number of individuals are prospectively identified (usually one or two). The investigational antisense oligonucleotide should be from a well-characterized chemical class for which there is substantial nonclinical information and clinical experience that is publicly available or to which the sponsor-investigator (hereafter referred to as sponsor) has a right of reference.