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After the recent high precision determinations of $V_{us}$ and $V_{ud}$, thefirst row of the CKM matrix shows more than $4\sigma$ deviation from unitarity.Two possible scenarios beyond the Standard Model can be investigated in orderto fill the gap. If a 4th quark $b'$ participates in the mixing, with $\vertV_{ub'} \vert \sim0.04$, then its mass should be no more than 6 TeV or so. Adifferent solution can come from the introduction of the gauge horizontalfamily symmetry acting between the lepton families and spontaneously broken atthe scale of about 6 TeV. Since the gauge bosons of this symmetry contribute tomuon decay in positive interference with Standard Model, the Fermi constant isslightly smaller than the muon decay constant so that unitarity is recovered.Also the neutron lifetime problem, that is about $4\sigma$ discrepancy betweenthe neutron lifetimes measured in beam and trap experiments, is discussed inthe light of the these determinations of the CKM matrix elements.

The CKM unitarity problem: A trace of new physics at the TeV scale?