Its association with the histone methyltransferase MLL2/MLL3 complex is suggesting a role in epigenetic transcriptional activation. However, in association with PAXIP1/PTIP is proposed to function at least in part independently of the MLL2/MLL3 complex. Proposed to be recruited by PAXIP1 to sites of DNA damage where the PAGR1:PAXIP1 complex is required for cell survival in response to DNA damage independently of the MLL2/MLL3 complex (PubMed:19124460). However, its function in DNA damage has been questioned (By similarity). During immunoglobulin class switching in activated B-cells is involved in transcription regulation of downstream switch regions at the immunoglobulin heavy-chain (Igh) locus independently of the MLL2/MLL3 complex (By similarity). Involved in both estrogen receptor-regulated gene transcription and estrogen-stimulated G1/S cell-cycle transition (PubMed:19039327). Acts as transcriptional cofactor for nuclear hormone receptors. Inhibits the induction properties of several steroid receptors such as NR3C1, AR and PPARG; the mechanism of inhibition appears to be gene-dependent (PubMed:23161582). Component of the KMT2 family MLL2/MLL3 complex (also named ASCOM complex), at least composed of the HMTs KMT2D and/or KMT2C, the common subunits ASH2L, RBBP5, WDR5 and DPY30, and the complex type-specific subunits PAXIP1/PTIP, PAGR1, NCOA6 and KDM6A; PAXIP1 is required for the association with the MLL2/MLL3 complex (PubMed:17500065). Forms a constitutive complex with PAXIP1/PTIP independently of the MLL2/MLL3 complex (By similarity). Interacts with NCOA1, ESR1, NR3C1, AR (PubMed:19039327,PubMed:19124460, PubMed:23161582). Ubiquitously expressed. The terminology of MLL proteins in mammalia is not consistent also concerning the terminology of MLL protein-containing complexes. The decribed MLL2/MLL3 complex is commonly described as MLL3/MLL4 complex in literature.