Ethical approval for this study was obtained from the Institutional Animal Care and Use Committee of Hoshi University (approval no. P21-039). A total of 8 female BALB/c mice (weight, 18–20 g; age, 8 weeks; Sankyo Labo Service Corporation, Inc.) were housed at 24°C and 55% humidity under 12/12-h light/dark cycle (lights on at 8:00 a.m.) with food and water ad libitum. siRNA lipoplexes with 20 µg Cy5-siRNA were administered intravenously to mice via the lateral tail vein (n=1/siRNA lipoplex). At 1 h post-injection of siRNA lipoplexes, mice were sacrificed via cervical dislocation; death was confirmed by cessation of heartbeat. Tissue (lung, heart, liver, spleen, and kidney) was analyzed by Cy5 fluorescence imaging using NightOWL LB981 NC100 system (Berthold Technologies GmbH & Co. KG), as previously described (21 (link)). The images were analyzed using IndiGo2 software (version 2.0.1.0) provided with the in vivo imaging system (Berthold Technologies). Following fluorescence imaging, tissue samples were frozen on dry ice and sliced into 16 µm sections. The localization of Cy5-siRNA was examined using a fluorescent microscope (Eclipse TS100-F; Nikon Corporation) with optical filter Cy5 HQ (excitation, 620/60 nm; dichroic mirror, 660 nm; emission, 700/75 nm; Nikon Corporation).