A Study of Mavorixafor in Participants With Congenital Neutropenia and Chronic Idiopathic Neutropenia Disorders

Key Inclusion Criteria: For all participants (Parts 1 and 2): * Sign the informed consent form (ICF) and be willing and able to comply with the protocol. * Weigh ≥15 kg * Agree to use a highly effective form of contraception if sexually active. * Participants may be eligible for the study whether they are on or off granulocyte-colony stimulating factor (G-CSF) treatment. * Note: Participants who are on G-CSF must be on a stable dose for ≥14 days prior to the Baseline visit and should not have an ANC ≥10,000 cells/μL. * Note: Participants who are not on G-CSF must be off for ≥14 days prior to the Baseline visit and have an ANC ≤1000 cells/µL at the Screening visit. * Note: Participants with Shwachman-Diamond syndrome, Cohensyndrome, and warts, hypogammaglobulinemia, infections and myelokathexis syndrome are eligible. Other types of chronic neutropenic disorders may also be eligible for enrollment upon discussion and approval with Sponsor and Study Medical Monitor. * Have been diagnosed with chronic neutropenia for ≥6 months prior to the Screening visit that is not attributable to medications, active or recent (≤3 months) infections, or malignant cause. Part 2 only: * Participants enrolled in the study before implementation of Protocol Version 8.0 must have completed Part 1 and exhibited a positive response to treatment. * Participant has a history of symptomatic chronic neutropenia confirmed by the Investigator. Key Exclusion Criteria (Parts 1 and 2): * Known syst…