Our genes are present in pairs in all the cells of our body. We inherit one gene of the pair from our mother and the other is inherited from our father. A spelling mistake within a gene is called a variant (also sometimes referred to as a mutation).
We usually need both copies of the HIST1H1E gene to be working to grow and develop. In Rahman syndrome, there is a variant in one of the gene pair and so it does not work optimally. A specific sort of variant, called a protein truncating variant, within a specific region of the gene causes Rahman syndrome.