  Recent data on the transverse single spin asymmetry $A_N$ measured by the
STAR Collaboration for $p^\uparrow \, p \to W^\pm/Z^0 \, X$ reactions at RHIC
allow the first investigation of the Sivers function in Drell-Yan processes and
of its expected sign change with respect to SIDIS processes. A new extraction
of the Sivers functions from the latest SIDIS data is performed and a critical
assessment of the significance of the STAR data is attempted.
