MED12 and leiomyosarcoma: Moreover, MED12 mutated UL but not those with HMGA2 rearrangements apparently display strong growth disadvantages in vitro [6] while, in rare cases, they may undergo malignant transformation following an UL-STUMP (smooth muscle tumor of uncertain malignant potential)-leiomyosarcoma sequence as witnessed by leiomyosarcomas and STUMP carrying MED12 mutations [7-11].