An open-label, multi-center, phase I/II study to assess the safety, tolerability and efficacy of DFT383 in pediatric participants with nephropathic cystinosis, followed by a long-term extension phase. The purpose of this clinical study is to assess safety, tolerability, and efficacy of DFT383 in participants aged 2 to 5 years with nephropathic cystinosis. The study consists of a Core Phase and a long-term Extension Phase. DFT383 is a cellular gene therapy. This study includes an active arm (Cohort 1) of participants treated with study treatment DFT383 and a concurrent reference arm (Cohort 0). Participants in Cohort 0 will not receive study treatment and will only participate in the Core Phase of the study. The study is not randomized and Cohort 0 aims to collect prospective and concurrent data in this rare disease.