  Heavy quark structure functions from HERA provide a direct handle on the
medium and small-x gluon PDF. In this contribution, we discuss ongoing progress
on the implementation of the FONLL General-Mass scheme with running heavy quark
masses, and of its benchmarking with the HOPPET and OpenQCDrad codes, and then
present the impact of the recently released combined HERA charm production
cross sections in the NNPDF2.3 analysis. We find that the combined charm data
contribute to constraining the gluon and quarks at small values of Bjorken-x.
