Cubic InN samples were grown on MgO (001) substrates by gas source molecular beam epitaxy (GSMBE). In general, we find that InN directly deposited onto the MgO substrate results in polycrystalline or columnar films of hexagonal symmetry. We find that adequate conditions to grow the cubic phase of this compound require the growth of an initial cubic GaN buffer interlayer (β-tGaN) on the MgO surface. Subsequently, the growth conditions were optimized to obtain good photoluminescence (PL) emission. The resultant InN growth is mostly cubic, with very small hexagonal inclusions, as confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies. Good crystalline quality requires that the samples to be grown under rich Indium metal flux. The cubic β-tInN/GaN/MgO samples exhibit a high signal to noise ratio for PL at low temperatures (20 K). The PL is centered at 0.75 eV and persists at room temperature.