  A simultaneous NLO QCD fit of the parton density functions (PDFs) of the
proton and of the strong coupling $\alpha_{S}(M_{Z})$ is presented. The
analysis is based on combined H1 and ZEUS inclusive DIS cross section
measurements together with inclusive jet cross section measurements provided by
both H1 and ZEUS collaborations. The inclusion of the jet data significantly
reduces the correlation between the gluon PDF and the strong coupling,
improving the precision of the gluon PDF and providing an accurate
determination of $\alpha_{S}(M_{Z})$ using DIS data from HERA only.
