NK cell deficiency (NKD) is a subset of primary immunodeficiency diseases/inborn errors of immunity (IEI) in which the NK cell abnormality represents the main clinical immunodeficiency. Patients with abnormal NK cells are susceptible to lethal virus infections and certain cancers, offering us a unique window into how these critical immune cells work. For over 15 years we have cared for and investigated these complex patients, applying genomic techniques to discover causative genes and illuminate NK cell biology. With this application, we aspire to renew our coordinated NKD discovery program, with the ultimate goals of understanding how to care for these patients as well as how to best use NK cells therapeutically. Over the past 5 years of our program, we have defined 2 new genetic causes of NKD and 8 new causes of IEI that affect NK cells. We established and grew an international referral network for NKD patients, honed methods to clinically and immunologically define these rare patients, matured our genomic evaluation/discovery pathways, and optimized patient-focused functional genomics and NK cell biological techniques, all to advance progress in understanding NKD. At present, we have 156 patients enrolled in our NK cell evaluation and research (NEAR) program at Columbia: of these, 70 have undergone exome sequencing (ES) at Baylor College of Medicine, 13 have found genetic solutions for their disease, 11 have a promising gene identified, and 36 remain unsolved. During this renewal period, we will build on our successful momentum, adding new NKD patients to our pipeline, clinically and immunologically defining their disease through the use of databases, advanced biostatistical techniques and research level phenotypic and functional assessments (Aim 1), adding new genomic discovery and analytic techniques like whole genome sequencing and RNA sequencing to bring clarity to the patients whose NKD genes remain ?unsolved? (Aim 2), and applying cutting-edge functional genomics and NK cell biological techniques to demonstrate the impact and relevance of the gene mutations we discover (Aim 3). We capitalize on strong, long-standing collaborations both within and beyond the field of Immunodeficiency in order to best define how the gene mutations we identify impact how NK cells function in human health. In so doing, we aim to not only better diagnose and care for these complex patients, but to better understand how NK cells protect humans from viruses and cancer.