Inclusion Criteria:
* Male or female participants aged 4 years ≤ age \< 18 years.
* Severe hypertriglyceridemia (sHTG), defined as a triglyceride (TG) level ≥ 500 mg/dL.
* Confirmed diagnosis of genetically inherited FCS via genetic testing, or clinically diagnosed FCS plus persistent chylomicronemia.
* Failure to achieve adequate TG control, For participants under 8 years of age, the investigator determine at their discretion whether prior lipidlowering therapy has been administered.
* Participants aged 6 years and above must sign the informed consent form themselves; for participants under 18 years of age, their parent/legal guardian must sign the informed consent form. (Participants under 6 years of age are exempt from signing the written informed consent form).
* Female participants of childbearing potential must have a negative result on serum pregnancy testing.
Exclusion Criteria:
* Currently participating in other interventional clinical studies, or having an insufficient washout period of less than 5 half-lives or 30 days (whichever is longer) since the last administration of other investigational drugs.
* Used antisense oligonucleotide (ASO)-based or small interfering RNA (siRNA)-based lipid-lowering drugs targeting APOC3 within 3 months prior to study drug administration.
* History of acute pancreatitis within 1 month before dosing.
* Patients who underwent major surgery within 3 months prior to study drug administration and are judged by the investigator as unsuitable for receiving the study drug, due to potential intolerance to adverse events such as cytokine release storm.
* ALT or AST ≥2 × ULN
* Total bilirubin ≥1.5 × ULN
* eGFR \<30 mL/min/1.73 m²
* Random urine albumin-to-creatinine ratio (UACR) \>30 mg/g, or urine protein is ≥ 2+
* HbA1c ≥9%
* Coagulation function abnormalities judged by the investigator as unsuitable for CS-121 administration.
* Positive results for HBsAg, dual positivity for HCV antibody and RNA, positive for HIV, or positive for Treponema pallidum infection.
* Known major organ diseases, mental disorders, Cushing's syndrome, hypothyroidism, history of lymphoproliferative disorders, or malignant tumors in any organ system, which are judged by the investigator as unsuitable for study participation due to potential intolerance to adverse events such as cytokine Release-Storm.
* Concomitant medications/treatments judged by the investigator to affect lipid metabolism, liver and kidney function, coagulation function, or interfere with the efficacy evaluation of the study drug.
* Patients of childbearing potential who are planning pregnancy, breastfeeding, or have fertility plans.
* History of hypersensitivity to any study drug, its excipients, or drugs of similar chemical classes.
* Other medical conditions or comorbidities that, in the investigator's opinion, may interfere with study compliance or data interpretation.
