Prothymosin alpha is a nuclear protein.

The cellular location of the so-called 'thymic hormone' prothymosin alpha has been studied by microinjection into the cytoplasm of Xenopus oocytes, followed by separate monitoring of nuclear and cytoplasmic concentrations. It is shown that prothymosin alpha migrates to the nucleus at a rate comparable to that of histone H1. Prothymosin alpha cannot therefore be a hormone in the usual sense of the word.