  Although the proton was discovered about 100 years ago, its spin structure
still remains a mystery. Recent studies suggest that the orbital angular
momentum of sea quarks could significantly contribute to the proton's spin. The
SeaQuest experiment, which recently completed data collection, probed the
unpolarized light quark sea distributions of the proton using the Drell-Yan
process. Its successor, the SpinQuest (E1039), will access the $\bar{u}$ and
$\bar{d}$ Sivers functions using polarized NH$_3$ and ND$_3$ targets. A
non-zero Sivers asymmetry, observed in SpinQuest, would be a strong indication
of non-zero sea-quark orbital angular momentum. The SpinQuest experiment can
also probe the sea quark's transversity distribution, which is relevant for the
determination of proton's tensor charge. Recent study suggests that sea-quarks
might contribute significantly to deuteron's tensor polarized structure
functions. This can be further probed in SpinQuest using tensor polarized
ND$_3$ target. The current status and future plan of the experiment are
presented.
