The goal of this clinical trial is to learn if haploidentical hematopoietic cell transplantation combined with an unrelated cord blood unit (haplo-cord HCT) works to treat acute T cell lymphoblastic leukemia (T-ALL). It will also learn about the safety of the transplantation. The main questions it aims to answer are:

Dose co-infusion of cord blood in haploidentical hematopoietic cell transplantation (haplo-HCT) lower the rate of relapse? What medical problems do participants have when having haplo-cord HCT? Researchers will compare haplo-cord HCT to haplo-HCT to see if haplo-cord HCT works to treat T-ALL.

Participants will be infused an unrelated cord blood unit at the same day of haploidentical graft infusion.

Inclusion Criteria:

• Patients with acute T cell lymphoblastic leukemia
• With available minimal residual disease parameters assessed by flow cytometry and/or quantitative polymerase chain reaction
• Willing to undergo haploidentical hematopoietic cell transplantation and having a suitable haploidentical donor
• With Eastern Cooperative Oncology Group (ECOG) performance status of 0-3
• Signing an informed consent form, having the ability to comply with study and follow-up procedures

Exclusion Criteria:

• With other malignancies
• Failing to acquire a suitable unrelated cord blood unit
• With a previous history of autologous hematopoietic cell transplantation, allogeneic hematopoietic cell transplantation or chimeric antigen receptor T cell therapy
• With uncontrolled infection intolerant to haploidentical hematopoietic cell transplantation
• With severe organ dysfunction
• In pregnancy or lactation period
• With any conditions not suitable for the trial (investigators' decision)