  Recent measurements on high-order harmonic generation (HHG) from N$_2$
aligned perpendicular to the driving laser polarization [B. K. McFarland {\it
el al}, Science {\bf 322}, 1232 (2008)] have shown a maximum at the rotational
half-revival. This has been interpreted as the signature of the HHG
contribution from the molecular orbital just below the highest occupied
molecular orbital (HOMO). By using the recently developed quantitative
rescattering theory combined with accurate photoionization transition dipoles,
we show that the maximum at the rotational half-revival is indeed associated
with the HOMO-1 contribution. Our results also show that the HOMO-1
contribution becomes increasingly more important near the HHG cutoff and
therefore depends on the laser intensity.
