In addition, some archaea do produce nucleosomes from proteins homologous to eukaryotic histones. Within eukaryotic chromosomes, chromatin proteins, such as histones, compact and organize DNA. The histone variant H2AX constitutes about 10% of the H2A histones in human chromatin. This immediately halts histone production and prevents a toxic buildup of free histones. These "minor histones" usually carry out specific functions of the chromatin metabolism. Archaeal histones may well resemble the evolutionary precursors to eukaryotic histones. Nucleosomes contain DNA and proteins called histones. Epigenetic control is often associated with alternative covalent modifications of histones. Protein methylation has been most studied in the histones. The lysine amino acid attached to the end of the histones is positively charged. Cancer can be caused in a variety of ways due to differential methylation of histones. Only some archaeal histones have tails. H1 is less conserved than core histones. Alternatively, transcription factors can bind enzymes that modify the histones at the promoter. These processes load newly synthesized histones onto DNA.