Molecular model showing the Fs12 fragment of follistatin (blue) bound to an activin A dimer (white). Binding by follistatin inhibits activin A. Activin A has numerous growth stimulating roles throughout the body. It has an effect on the development of a number of organs and plays a role in the regulation of tissue repair, metabolism, the menstrual cycle and the immune system. It has been shown that activin A inhibits the activity of natural killer immune cells against cancerous tumours. Inhibiting activin A with follistatin improved the ability of the immune system to fight tumours.