[New ways in interdisciplinarity: internal medicine meets human genetics : When frequent conditions become rare and rare conditions become frequent].

Molecular findings have led to a deeper understanding of the pathophysiology of numerous diseases and now form the basis for targeted treatment. An example of this is chronic myeloid leukemia (CML). The identification of the BCR::ABL1 translocation enabled the development of specific tyrosine kinase inhibitors. While the median survival time used to be 4 years, CML is now often a chronic disease with a near-normal life expectancy thanks to targeted treatment; however, most tumor diseases are more complex at the molecular level. Advances in genome analysis enable increasingly more refined molecular characterization. Common tumor diseases are thus divided into increasingly smaller molecularly distinct segments, which become rarer as individual entities but can also be treated in a more targeted and individualized manner. The concept of personalized medicine is manifested in molecular tumor boards. A similar approach can start from rare genetic syndromes. In this case, understanding the underlying pathophysiology does not necessarily lead to a causal treatment of the syndrome itself but it does enable new treatment options for frequent diseases. An example is thyroid hormone resistance (RTH)β. Findings on the effect of thyroid hormone receptor β in the liver have contributed to the development of analogues that can now be used specifically to treat metabolic dysfunction-associated steatohepatitis (MASH) with fibrosis and can possibly also have the potential to reduce the progression of liver cirrhosis. Overall, close cooperation between human genetics and internal medicine can substantially contribute to an improvement in treatment success. A better understanding of molecular disease mechanisms enables an increasingly more precise, individualized and effective treatment.