SMARCA4 and cancer: The most frequent SWI/SNF complex subunit alteration in thorax, lung, pleural and mediastinal neoplasms is SMARCA4. SMARCA4 - deficient undifferentiated tumors were recently recognized as a new entity in the WHO classification of thoracic tumors and are defined as malignant neoplasms with an undifferentiated or rhabdoid phenotype and deficiency of SMARCA4 (BRG1)5.