This is a Phase I/II, open label, single center study to assess the safety and tolerability of EXG34217 in bone marrow failure patients with telomere biology disorders.

This is a Phase I/II, open label study in up to 12 subjects with telomere biology disorders with bone marrow failure. The study is open to all participants regardless of gender or ethnicity. Subjects who are enrolled but not evaluable will be replaced.

Subjects will sign a consent form prior to any study related procedure and will complete baseline screening assessments. Subjects for this study will not require any preparative regimen such as chemotherapy or radiation.

The study will be conducted in three parts

• Peripheral blood mononuclear cells (PBMNCs) collection; mobilization and apheresis,
• Ex vivo cell processing
• Processed cell infusion and post-infusion safety monitoring,
• Follow-up (Week 2, 3,4,5, Months 1, 2,3,4,5,6,9 and 12)

Inclusion Criteria:

• Age 12 years and above
• Mild or moderate bone marrow failure defined by satisfying specific conditions.
• Diagnosis of telomere biology disorders

Exclusion Criteria:

• Women of child bearing potential or breastfeeding.
• Patients with cancer who are on active chemotherapeutic treatment.
• Patients with severe bone marrow failure.
• Clonal cytogenetic abnormalities associated with MDS or AML on bone marrow examination.
• Uncontrolled bacterial, viral or fungal infections.
• Prior allogeneic marrow or stem cell transplantation.
• Patients who are not eligible for G-CSF and plerixafor dosing.
• Patients who are not eligible for the apheresis.
• Patients currently taking or have taken danazol and androgens within 60 days prior to Day 1.
• Patients with any other clinically relevant acute or chronic diseases which could interfere with the patients' safety during the trial, expose them to undue risk, or which could interfere with study objectives.
• Patients who have participated in another clinical trial with an investigational drug within the previous 30 days.