  Single spin asymmetries in semi-inclusive deep-inelastic scattering off
transversely polarized targets give information on, among other fascinating
effects, a pseudo time-reversal odd parton distribution function, the 'Sivers
function'. In this proceeding we review the extractions of this function from
HERMES and COMPASS data. In particular, the HERMES pion and kaon data suggest
significant sea-quarks contributions at $x \simeq 0.15$ to the Sivers effect.
We present a new fit that includes all relevant sea quark distributions and
gives a statistically satisfactory overall description of the data, but does
not describe ideally the $K^+$ data from HERMES. We argue that measurements of
the pion- and kaon Sivers effect at CLAS12, and COMPASS, will clarify the
situation.
