Comment on "Proton Spin Structure from Measurable Parton Distributions" by Ji, Xiong and Yuan (PRL109, 152005 (2012))

We show that the recent claim that the expression 1/2 \int x dx [ H_q (x,0,0) + E_q(x,0,0)], involving the generalized parton distributions H and E, measures the transverse angular momentum of quarks in a transversely polarized nucleon, is incorrect.