TBX20 gene encodes a transcription factor that is essential for heart development. Recent experimental data implicate loss of TBX20 function in the development of left ventricular noncompaction cardiomyopathy (LVNC) but supporting clinical and genetic data in affected families is scarce. We report a series of four families with TBX20 loss-of-function variants that segregate with LVNC. This family data establishes a definitive association between TBX20 loss-of-function variants and LVNC and expands the TBX20 variant spectrum. Our results emphasize the importance of including TBX20 in genetic testing for LVNC to improve the diagnostic yield of genetic testing. (By Dr Richard D Bagnall, https://jmg.bmj.com/content/early/2023/09/01/jmg-2023-109455 )