15/15. Spin(11,1) string theory is:

1. Motivated by the fact that Spin(10) chirality equals Dirac chirality. From this, all else follows.

2. Not a direct product of internal and spacetime manifolds.

3. 11+1 dimensional, nontrivially accommodating 10 compact internal dimensions and 3+1 large spacetime dimensions.

4. Weak and color gauge fields are carried by bosonic open strings whose ends attach to D4- and D6-subbranes of the internal fermionic D10-brane.

5. The complex fermionic D10-brane becomes a Calabi-Yau five-fold on Pati-Salam symmetry breaking to the standard model.

6. A 26d bosonic string theory compactified to 12d on a 14d maximal self-dual torus of SU(8)×SU(8) (like heterotic theory compactified to 10d on the 16d maximal self-dual torus of E₈×E₈).

7. Not supersymmetric, so does not predict unobserved superpartners.

8. Anomaly-free, modular-invariant.

9. Are there Calabi-Yau five-folds with Euler characteristic ±6, giving three generations?

10. Tachyonic. It decays through tachyonic Higgs fields down to the present time, where the vacuum energy (dark energy) is apparently stable over cosmological timescales.

11. Spin(11,1) may already be ruled out because the predicted energy scale of grand unification, 10¹⁵ GeV, is less than the lower limit of 4×10¹⁵ GeV inferred for Spin(10) models by King et al. (2021) from the Super-Kamiokande lower limit of 1.6×10³⁴ yr on the proton lifetime. A more careful calculation needs to be done that follows the symmetry breaking chain of Spin(11,1) rather than Spin(10).