  The first measurement of the top quark pair production cross section
($\sigma_{\rm{t}\bar{\rm{t}}}$) in proton-proton collisions at $\sqrt{s} =
5.02$ TeV is reviewed. The data have been collected by the CMS experiment at
the LHC and analyzed considering events with at least one charged lepton. The
extraction of $\sigma_{\rm{t}\bar{\rm{t}}}$ can be used to constrain the gluon
distribution function (PDF) at large longitudinal parton momentum fraction and
to establish experimentally the relation between the top-quark mass as
implemented in Monte-Carlo generators and the Lagrangian mass parameter. The
impact of the measurement on the determination of the gluon PDF is illustrated
through a quantum chromodynamic analysis at next-to-next-to-leading order and
the result is furthermore put in context with other top quark measurements in
different phase space regions. The measurement has paved the way for the first
observation of top quark production in nuclear collisions and the subsequent
study of modifications induced on the bound gluon PDF.
