What is the fine-grained topic of the following text: You know our dramatic story of Nic being diagnosed with an ultra-rare genetic disorder of the immune system called X-linked lymphoproliferative syndrome 2 that was due to a change in the XIAP gene (XIAP deficiency). Only return the topic.
Nic's diagnosis with X-linked lymphoproliferative syndrome 2 due to a change in the XIAP gene.