SMARCB1/ INI1 is one of the core subunit proteins of the ATP‐dependent SWI/ SNF chromatin remodeling complex, and is identified as a potent and bona fide tumor suppressor. Interactions have been demonstrated between SMARCB1/ INI1 and key proteins in various pathways related to tumor proliferation and progression: the p16‐ RB pathway, WNT signaling pathway, sonic hedgehog signaling pathway and Polycomb pathway. Initially, no detectable SMARCB1/ INI1 protein expression was found in malignant rhabdoid tumor cells, whereas all other kinds of tumor cells and non‐tumorous tissue showed SMARCB1/ INI1 protein expression. Therefore, immunohistochemical testing for the SMARCB1/ INI1 antibody has been considered useful in confirming the histologic diagnosis of malignant rhabdoid tumors. However, recently, aberrant expression of SMARCB1/ INI1 has been found in various tumors such as epithelioid sarcomas, schwannomatosis, synovial sarcomas, and so on. In addition, it has been reported that aberrant expression can be classified into three patterns: complete loss, mosaic expression and reduced expression. Although the various pathways related to mechanisms of tumorigenesis and tumor proliferation are complexly intertwined, the clarification of these mechanisms may contribute to therapeutic strategies in SMARCB1/ INI1‐deficient tumors. In terms of pathological classifications, SMARCB1/ INI1‐deficient tumors may be re‐classified by genetic backgrounds.