This is a single-arm, open-label, single-dose, dose-escalation trial that plans to enrol 3-18 patients with transfusion-dependent β-thalassaemia (TDT) or sickle-cell disease (SCD). Its primary aims are to evaluate the safety and tolerability of a single administration of YOLT-204 and to obtain preliminary data on its effect on plasma fetal-haemoglobin levels. The main-study screening period may last up to 60 days; the treatment day is Day 0 (D0). Safety follow-up continues through Week 52 post-dose. After completion of the main study, participants will enter long-term follow-up extending to 15 years post-dose.