TDE1, tumor differentially expressed protein 1, is a multi-pass membrane protein also known as serine incorporator 3. Also found on the membranes of the Golgi apparatus within cells, TDE1 is highly expressed in neuronal populations but is also found in thymus, kidney, liver and testis. Expression levels of TDE1 in tumors can be as much as tenfold the amount found in normal tissue of the same type. This increased expression implicates TDE1 as being involved in the cellular transformation from normal to malignant tissue. It is believed TDE1 contributes to oncogenesis by partially protecting cells from serum starvation and etoposide-induced apoptosis.