Many of the individuals with variants in NAA15 have a mutation that causes the protein to either be much smaller or to not exist at all, at least as expressed from that one gene copy. However, it is important to remember that humans have two copies of each chromosome, so these individuals have a normal copy of the NAA15 gene. That means that each individual might express ~50% of normal NAA15, and this might help to explain why the clinical features in the syndrome are quite variable. Future work with cell lines and mouse models might help to explain this further.
NAA15