PHF1 gene, mapping on chromosome band 6p21, encodes a Polycomb group protein, a protein with significant sequence similarity to Drosophila Polycomblike. The protein is a component of a histone H3 lysine-27 (H3K27)-specific methyltransferase complex, and functions in transcriptional repression of homeotic genes. PHF1 has several biochemical functions, for example, methylated histone binding, protein binding, transcription factor activity, sequence-specific DNA binding. Some of the functions are cooperated with other proteins, some of the functions could acted by PHF1 itself. The protein is also recruited to double-strand breaks, and reduced protein levels results in X-ray sensitivity and increased homologous recombination. Diseases associated with PHF1 include Endometrial Stromal Sarcoma and Endometrial Stromal Tumor. PHF1 gene was first identified as being rearranged in low-grade endometrial stromal sarcoma (LG-ESS). The gene is known to recombine with four partners through different chromosomal rearrangements within the same tumor type. It was found fused with JAZF1 (from 7p15; through an unbalanced 6;7-translocation), with EPC1 (from 10p11; through a 6;10-translocation), with MEAF6 (from 1p34; through a 1;6-translocation), and, the so far latest addition to the list, with BRD8 (from 5q31, through a 5;6-rearrangement).