In recent years, oligonucleotide therapeutics (ONT), such as antisense oligonucleotides and siRNA therapeutics, have been attracting attention as a new modality to treat intractable and genetic diseases. The mechanisms of ONT-mediated toxicity can be conceptually divided into hybridization-dependent effects and hybridization-independent effects. The hybridization-dependent effects, also called off-target effects, are potentially caused by inadvertent binding of ONT to unintended RNAs. Toxicity induced by off-target effects could not be evaluated by conventional animal studies theoretically because of differences in the genome sequence between humans and other animals. Consequently, to predict toxicity induced by off-target effects, assessment of off-target effects with in silico analysis using a human RNA database and in vitro expression analysis using human cells has been proposed. The toxicity via hybridization-independent effects is likely to be due to interactions between the ONT and cellular proteins and it is assumed that the toxicity can be evaluated by animal experiments, though species differences are also observed in the hybridization-independent mechanism (e.g. innate immune activation via Toll-like receptors). Against this background, we develop and evaluate methods for safety assessment of ONT using human-derived cells and humanized animals. In this talk, I would like to outline the basis of ONT and discuss the IVIVE and species differences.