This is a Phase 1/2, multicenter, open-label study to evaluate the safety and efficacy of BEAM-201 in patients with relapsed/refractory T-ALL or T-LL. This study consists of Phase 1 dose-exploration cohorts, Phase 1 dose-expansion cohort(s), a Phase 1 pediatric cohort (will enroll patients ages 1 to < 12 years), and a Phase 2 cohort.

Key Inclusion Criteria:

1. Ages 18 to ≤ 50 years.

2. Ages ≥ 1 year to < 18 years, after health authority approval.

3. T-ALL/T-LL that is CD7-positive (defined as at least 20% of blasts positive for CD7 by flow cytometry or immunohistochemistry based on assessment of the study site's CLIA [Clinical Laboratory Improvement Amendments of 1988] certified facility) in second or greater relapse, first relapse post-transplant relapse, or chemotherapy-refractory disease. Specifically:

   1. Second or greater relapse or post-transplant relapse, defined as:

      • BM with ≥ 5% lymphoblasts by morphologic assessment or evidence of extramedullary disease at screening after second documented CR; OR
      • Flow cytometric confirmation of relapsed T-ALL of at least 0.1% after second CR documented to have been MRD negative < 0.1%; OR
      • Any detectable relapsed disease post-allogeneic HSCT with flow cytometric confirmation of T-ALL of at least 0.1%; OR
      • Biopsy confirmed evidence of relapsed T-LL on lymph node biopsy after second CR; OR
      • Any detectable disease post-allogeneic transplant with biopsy confirmed evidence of T-LL on lymph node biopsy

   2. Refractory disease, defined as:

      • Primary refractory T-ALL or T-LL, defined as failure to achieve CR after induction chemotherapy, per investigator assessment and based on biopsy-confirmed evidence of residual T-ALL or T-LL; OR
      • Relapsed, refractory disease, defined as > 5% BM blasts or biopsy-confirmed evidence of residual TLL after 1 course of re-induction chemotherapy for patients who have relapsed after previously achieving a CR NOTE: Patients with mixed phenotype acute leukemia with T-cell dominant phenotype may be enrolled if the aforementioned criteria are met.

4. Eligible for myeloablative conditioning for and allogeneic HSCT based on the investigator's assessment with an available donor identified by a FACT accredited transplant center.

Key Exclusion Criteria:

1. CNS involvement meeting any of the following criteria: CNS-3 disease, progressive CNS involvement despite therapy, CNS parenchymal or cranial nerve lesions on imaging.
2. Clinically active CNS dysfunction or known history of irreversible neurological toxicity related to prior antileukemic therapy.
3. Receipt of prior CD7 targeted therapy.
4. Systemic antileukemic therapy intended to induce or maintain remission within 14 days prior to completion of screening.