Promotes beta-catenin's transcriptional activity. Involved in signal transduction through the Wnt pathway (By similarity). Binds to beta-catenin (CTNNB1), PYGO1 and PYGO2; the interaction with PYGO1 increases PYGO1 affinity to histone H3 methylated at 'Lys 4'. Belongs to the BCL9 family. Truncated N-terminus.