  We review the study the Wandzura-Wilczek relation for the structure function
g_2, with a particular attention on the connection with the framework of
Transverse Momentum Dependent factorization. We emphasize that the relation is
broken by two distinct twist-3 terms. In the light of these findings, we
clarify what can be deduced from the available experimental data on g_2, which
indicate a breaking of the order 20-40%, and how to individually measure the
twist-3 terms.
