 Evidence for this hypothesis comes from experiments showing that the timing of MBT can be sped up by adding extra DNA to make the nucleus larger, or by halving the amount of cytoplasm.The exact methods by which the cell achieves this control is unknown, but it is thought to involve proteins in the cytosol.In "Drosophila", the zinc-finger transcription factor Zelda is bound to regulatory regions of genes expressed by the zygote, and in zebrafish, the homeodomain protein Pou5f3 (a paralog of mammalian POU5F1 (OCT4) has an analogous role.