Histone lysine methyltransferases (KMTs) are key actors in the regulation of the cell's most critical functions, including but by no means limited to gene expression, DNA damage repair, and cell differentiation. Histone KMTs impart control over these processes by remodelling chromatin through the modification of histone N-tail lysine residues. While historically studied exclusively within the context of histones, emerging evidence points to a broader role for histone KMTs, specifically through the lysine methylation of non-histone proteins. In this thesis, I investigate non-histone lysine methylation activity of the KMT2-class enzyme MLL4, characterized by its monomethylation activity on histone H3K4 at distal enhancer regions and devoid of other known substrates. Through tandem shotgun and systematic approaches, I identify the histone KMT-associated regulatory protein CXXC finger protein 1 as a novel substrate of MLL4, and conduct in vitro investigation of the potential functional consequences of this methylation event.