Homozygosity mapping has power to identify a disease-causing gene in as few as 3 patients, and we have indeed identified the SLC34A2 gene in pulmonary alveolar microlithiasis and the OPTN gene in the amyotrophic lateral sclerosis both in 3 patients [9,10]. Here, SLC34A2 is linked to pulmonary alveolar microlithiasis.