WHIM syndrome (Warts, Hypogammaglobulinemia, Infections and Myelokathexis) is a rare immunodeficiency disorder caused by mutations in the CXCR4 chemokine receptor. Patients present with severe chronic neutropenia and lymphopenia due to aberrant retention of leukocytes in the bone marrow as a result of CXCR4 hyperactivation. There is no approved therapy for the treatment of pts with WHIM and current therapeutic options do not address the primary pathophysiology. X4P-001 is an oral, selective, small molecule, allosteric antagonist of CXCR4 signaling that is being evaluated as a targeted therapy for WHIM. It is hypothesized that X4P-001 will stimulate immune response by increasing neutrophil and lymphocyte mobilization from the bone marrow into the circulation and lead to improvement in various clinical symptoms of the disease. An interim result from Phase 2 of a Phase 2/3 study examining X4P-001 therapy in patients with WHIM. The primary study objectives are to evaluate safety and tolerability of X4P-001 in WHIM patients and determine the dose required to achieve a consistent increase in circulating neutrophils and lymphocytes for the Phase 3 portion of the study.