Every nucleated diploid cell in the body, with the exception of B and T cells of the immune system, has the same genome as its originating single fertilized egg. During development, this single cell differentiates into a complex multicellular organism composed of various cells and tissues each carrying out specialized functions. Although each cell contains a genome of data it needs to select the relevant information from this genetic blueprint to fulfil its own specific function. ‘Molecular interactions’ shows that proteins must be produced in the right place and at the right time. This requires regulation of gene expression in conjunction with a myriad of bio-molecular interactions to coordinate this.