DNA histones and gene transcription. Illustration of a stage in the transcription of DNA (deoxyribonucleic acid), with RNA (ribonucleic acid) at upper centre and upper left, and a transcription enzyme at centre right, acting on the P53 gene (a tumour suppression gene). The DNA strands coil around proteins called histones (blue). This forms circular structures called nucleosomes (several shown here). These histones have 'tails' sticking out for modification by attracting other proteins to attach to them. This process (involving histone factors such as histone methylation) affects the expression of the genes and is an example of epigenetics.