  We discuss an $SU(5)$ Grand Unified Theory (GUT) based on the 10d orbifold
$(\mathbb{T}^2)^3/(\mathbb{Z}_4\times\mathbb{Z}_2)$ plus three modular $S_4$
groups with moduli at respective fixed points $i,i+2,\omega=e^{2i\pi/3}$. The
resulting model has hierarchical quark and charged lepton mass matrices,
arising from a double weighton mechanism, and reproduces the highly predictive
Littlest Seesaw Mechanism in the neutrino sector. The down quark mass matrix
has an upper triangular form, contributing to CKM mixing, while the charged
lepton mass matrix has a lower triangular form with suppressed contributions to
PMNS mixing. The orbifold yields successful $SU(5)$ breaking with
doublet-triplet splitting of the Higgs multiplets.
