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Single spin asymmetries in semi-inclusive deep-inelastic scattering offtransversely polarized targets give information on, among other fascinatingeffects, a pseudo time-reversal odd parton distribution function, the 'Siversfunction'. In this proceeding we review the extractions of this function fromHERMES and COMPASS data. In particular, the HERMES pion and kaon data suggestsignificant sea-quarks contributions at $x \simeq 0.15$ to the Sivers effect.We present a new fit that includes all relevant sea quark distributions andgives a statistically satisfactory overall description of the data, but doesnot describe ideally the $K^+$ data from HERMES. We argue that measurements ofthe pion- and kaon Sivers effect at CLAS12, and COMPASS, will clarify thesituation.

Sivers effect at Hermes, Compass and Clas12