Abstract: The present invention relates to a polypeptide (BIG1) and variants thereof capable of enhancing the rate of cell-division of a microorganism or plant cell, as well as nucleic acid molecules encoding said polypeptides, vectors comprising said nucleic acid molecules and host cells transformed or transfected with said vectors and expressing said polypeptides. The BIG1 polypeptide which has been identified in the marine centric diatom Thalassiosira pseudonana, variants thereof and nucleic acids encoding these may be used in methods of enhancing the rate of cell-division of microorganisms, plant cells or plants which produce useful sub stances or exhibit useful properties, to increase the yield thereof.