  We discuss the impact of the charm quark mass in the CTEQ NNLO global
analysis of parton distribution functions of the proton. The $\bar{\rm MS}$
mass $m_c(m_c)$ of the charm quark is extracted in the S-ACOT-$\chi$
heavy-quark factorization scheme at ${\cal O}(\alpha_s^2)$ accuracy and found
to be in agreement with the world-average value. Impact on $m_c(m_c)$ of
combined HERA-1 data on semiinclusive charm production at HERA collider and
contributing systematic uncertainties are reviewed.
