STMN1 (Stathmin; also Op18, Pr22 and LAP18) is a 19 kDa member of the Stathmin family of proteins. It is a ubiquitously expressed cytoplasmic phosphoprotein that destabilizes microtubules during cell quiescence. Human STMN1 is 149 amino acids (aa) in length and the entire molecule forms a Stathmin domain that consists of a phosphorylatable regulatory sequence (aa 2 - 65), and an overlapping alpha-helical interaction region (aa 60 - 149).

Phosphorylation on Ser16, 25, 38 and 63 destabilizes STMN1 and allows for microtubule polymerization during the cell cycle. There is one potential isoform that shows a 38 aa substitution for aa 79 - 149. Full-length human STMN1 (aa 21 - 149) shares 99% aa identity with mouse STMN1.