The methyl lysine readers PHF (plant homeodomain finger) 20 (PHF20) and its homolog PHF20 Like 1 (PHF20L1) are known components of the NSL (non-specific lethal) complex that regulates gene expression through its histone acetyltransferase activity. In the current model, both PHF homologs coexist in the same NSL complex although this was not formally tested; nor have the functions of PHF20 and PHF20L1 regarding NSL complex integrity and transcriptional regulation been investigated. By performing an in-depth biochemical and functional characterization, we define the association and contributions of the two methyl lysine reader homologs PHF20 and PHF20L1 to the NSL complex.