Why do cells differ?Your cells come in all shapes and sizes. Different types of cell make different proteins according to the jobs they have to do. For example, only red blood cells contain the protein haemoglobin which carries oxygen around your body. Similarly, only cells in your eyes make proteins for detecting light. As well as these 'specialised' proteins, almost all your cells share a common set of 'housekeeping' proteins.
How do cells decide which proteins to make?
Almost all your cells have the same set of genes. These carry instructions for making every protein in your body. But any particular cell uses only a selected set of genes. If a gene is 'switched off', no protein will be made from that gene. Mechanisms for switching genes on and off are crucial to the functioning of every cell.