Hoyeraal Hreidarsson Syndrome

The goal of this clinical trial is to learn if a combination therapy of deoxycytidine (dC) plus deoxythymidine (dT) is safe in patients with telomere biology disorders. The main questions it aims to answer are: * Is the therapy safe with tolerable side effects in patients with telomere biology disorders? * Are problems with the bone marrow or blood or lungs changed after 6 months of dC+dT treatment in patients with telomere biology disorders? Participants will: * Take study drug by mouth three times daily for 24 weeks * Make approximately 2 visits to Boston Children's Hospital during the 24 weeks: once at the beginning of treatment and once at the end of treatment. * Go to a lab for a blood draw an additional 6 times during treatment. * Have 9 phone calls with a research nurse, including one 4 weeks after treatment ends. * Keep a diary to track doses of study drug that were taken or missed.

Phase II prospective trial to assess the rates of donor engraftment using reduced intensity conditioning (RIC) hematopoietic stem cell transplant (HSCT) and post-transplant cyclophosphamide (PTCy) for patients with primary immune deficiencies (PID), immune dysregulatory syndromes (IDS), inherited bone marrow failure syndromes (IBMFS), short telomere syndromes, Fanconi anemia, and non-Fanconi DNA double-strand break (DNA-dsb) repair disorder.