It had been naively believed that the quark sea in the proton was formed by quantum chromodynamics (QCD) processes that did not discriminate between up and down quarks. However, results of deep inelastic scattering of high energy muons on a proton and a deuteron targets by CERN-NMC showed that there are more d 's than u 's in the proton. The Gottfried sum measured by NMC was 0.235±0.026, which is significantly smaller than the expected value of 1/3. This means that d (x)- u (x) integrated over Bjorken x from 0 to 1.0 is 0.147±0.039, indicating a flavor asymmetry in the proton sea. Recent measurements using Drell–Yan scattering probed the flavor asymmetry of the proton. To leading order in the strong interaction coupling constant, α s, the Drell-Yan cross section is given by