Collectively, rare diseases affect millions of people worldwide. Understanding the molecular cause of rare disease has important implications for clinical management. However, although most rare diseases are suspected to be genetic in origin, the causal genes are not known in a majority of affected families. This study will use emerging technologies to better understand the molecular basis of rare genetic diseases. Long-read genome sequencing, a recent genetic testing technology, will help us to identify rare and complex genetic changes in individuals suspected to have harmful genetic variation. These findings will allow us to study how specific genes lead to congenital disorders and adult-onset cancer predisposition syndromes, genetic syndromes that increase the risk of developing specific types of cancers. This research will improve our understanding of normal and disease-causing genetic variation and help establish a foundation for the broader application of new technologies in the clinic.