De novo truncating mutations in the ASXL1 gene have been identified as a definitive cause of BOS (Hoischen et al. 2011; Abdel-Wahab et al. 2013; Bedoukian et al. 2018; Zhao et al. 2021; Lin et al. 2023). The gene discussed is ASXL1; the disease is Buschke-Ollendorff syndrome.