This clinical trial is focused on studying WHIM Syndrome, a rare genetic disorder that affects the immune system. The study is testing a treatment called Mavorixafor, which is taken as a capsule. The purpose of the study is to evaluate how effective Mavorixafor is in increasing the levels of certain white blood cells, called neutrophils, in people with WHIM Syndrome. The study also aims to assess the long-term safety and tolerability of the treatment.

The study is divided into two parts. In the first part, participants will be randomly assigned to receive either Mavorixafor or a placebo, which looks like the treatment but does not contain the active ingredient. This part of the study will help determine how well Mavorixafor works compared to the placebo. In the second part, all participants will receive Mavorixafor to further evaluate its safety over a longer period.

Throughout the study, participants will have regular check-ups to monitor their health and the effects of the treatment. This includes blood tests to measure the levels of neutrophils and other white blood cells, as well as assessments of any side effects. The study will help researchers understand if Mavorixafor can be a beneficial treatment for people with WHIM Syndrome.