Bcl-9 (B-cell lymphoma 9; also Protein legless homolog) is a 149kD transcriptional regulator that belongs to the Bcl-9 family of proteins. It is expressed in multiple tissues and serves to recruit Pygopus to the Wnt-pathway beta-catenin-TCF complex in the nucleus. Bcl-9 and Bcl-9-2 are considered evolutionary duplicates of Legless that perform the same task with different regulatory mechanisms. Human Bcl-9 is 1426aa in length. It contains one phosphothreonine and three phosphoserine sites, two poly-Pro regions (aa514-517 and 970-973), and one poly-Ala segment (aa900-903). There is one potential alternate start site at Met27, and a variant isoform exists that shows a four aa substitution for aa 1391-1426. Over aa1009-1328, human Bcl-9 is 96% aa identical to mouse Bcl-9. B cell cancers often have translocations at the 3’UTR region of the Bcl-9 gene.