Thomas Fox, MBChB, MD, PhD, dives into the complexities of treating primary immunodeficiencies with bone marrow transplants, highlighting the necessity for highly individualized patient assessments. He introduces two risk evaluation tools: the conventional HCTI score and the innovative IDDA score, designed specifically for rare conditions. By comparing these tools, Fox illustrates how the IDDA score can offer more nuanced insights, potentially reclassifying some patients’ transplant risks, and thus enhancing the precision of treatment strategies for these vulnerable patients.