Cytokines can be grouped into a family on the basis of sequence, functional and structural similarities. Tumor necrosis factor (TNF) (also known as TNF-alpha or cachectin) is a monocyte-derived cytotoxin that has been implicated in tumour regression, septic shock and cachexia. The protein is synthesised as a prohormone with an unusually long and atypical signal sequence, which is absent from the mature secreted cytokine. A short hydrophobic stretch of amino acids serves to anchor the prohormone in lipid bilayers. Both the mature protein and a partially-processed form of the hormone are secreted after cleavage of the propeptide. There are a number of different families of TNF, but all these cytokines seem to form homotrimeric (or heterotrimeric in the case of LT-alpha/beta) complexes that are recognised by their specific receptors.