Involved in signal transduction through the Wnt pathway. Promotes beta-catenin's transcriptional activity (By similarity). Binds to beta-catenin (CTNNB1), PYGO1 and PYGO2; the interaction with PYGO1 increases PYGO1 affinity to histone H3 methylated at 'Lys 4'. Detected at low levels in thymus, prostate, testis, ovary and small intestine, and at lower levels in spleen, colon and blood. A chromosomal aberration involving BCL9 is found in a patient with precursor B-cell acute lymphoblastic leukemia (ALL). Translocation t(1;14)(q21;q32). This translocation leaves the coding region intact, but may have pathogenic effects due to alterations in the expression level of BCL9. Several cases of translocations within the 3'-UTR of BCL9 have been found in B-cell malignancies. Belongs to the BCL9 family. It is uncertain whether Met-1 or Met-27 is the initiator.