BCL-6 interacting corepressor (BCOR) is a 1,755 amino acid protein that associates with histone deacetylases to transcriptionally repress BCL-6. BCOR mRNA is ubiquitously expressed in various tissues. However, the BCOR protein expression in adult tissue is limited to testis only. Recently, the BCOR rearrangement was detected in a subset of small blue round, spindle cell, and undifferentiated sarcomas, which results in overexpression of BCOR protein in these tumors, including small round cell sarcoma (~100%), clear cell sarcoma of kidney (100%), high-grade endometrial stromal sarcoma (100%) with YWHAE-NUTM2 translocation, and subset of synovial sarcoma (50%). The list of BCOR positive tumors is still expanding.