Histone proteins play a critical role in packaging genomic DNA and in epigenetic signaling. Since their initial characterization five decades ago, several histone variants have been identified and shown to be vital in various DNA-based transactions. For example, unlike histone H3.1 (canonical), histone H3.3 (variant) is deposited on chromatin throughout the cell cycle and in non-dividing cells. However, these two histones share over 95% sequence identity. Although nearly identical, both histone H3 proteins have been exquisitely conserved throughout evolution. I will present the results of our efforts to determine the structural underpinnings of the recognition and methylation of histone H3.1 by a novel histone reader and a dedicated SET domain lysine methyltransferase, respectively. Our findings suggest that histone H3.1 is vital for the mitotic inheritance of a heterochromatic mark during DNA replication and critical for DNA damage repair. Finally, I will provide structural and biochemical evidence showing that key mutations of a histone H3.1 reader are an important etiological factor for the progression of Sponastrime dysplasia.