Genetics and Duchenne Every organism is made up of cells, described as the building blocks of life, carrying out important functions involved in the survival of an organism. Within each cell is a command centre (nucleus), containing instructions (genes) to make proteins. It is believed that humans have around 20,000 genes in total. Genes are found on chromosomes, with humans having 23 pairs of chromosomes (46 in total). Chromosomes are made from DNA, with DNA containing all the genetic information of an individual, with genes being small parts of DNA. The 23rd chromosome pair is a sex chromosome. Females have 2 X chromosomes and males have 1 Y and 1 X chromosome. You inherit one chromosome from each pair, one from your mother and one from your father. Duchenne is described as an X-linked condition mostly affecting boys, alterations of the dystrophin gene on the X-chromosome are inherited from the mother. Genes are made up of exons and introns, exons containing messages to make proteins, important in the structure, function and regulation of the body. Introns do not contain information for making proteins. When proteins are made introns are left out leaving exons. In order to make proteins, exons are read in groups of three’s (triplets) in a ‘reading frame’ in order to produce a functioning protein. The dystrophin gene is the largest human gene made of 79 exons, providing instructions for making a protein called dystrophin. Sometimes genes become altered (mutated) in such a way that certain proteins are not produced correctly. In Duchenne the dystrophin gene is altered, resulting in the dystrophin protein not being produced correctly. This leads to problems affecting the muscles, however cells in the brain (nerve cells) can be affected, leading to problems with behaviour and learning in some individuals. Figure 1 - The dystrophin gene is the largest known human gene made up of 79 exons. Image by Annemieke Aartsma-Rus, from Exon skipping therapy for Duchenne muscular dystrophy, Leiden University Medical centre, 2008.