  It is here explained how the Green-Schwarz superstring theory arises from
Matrix String Theory. This is obtained as the strong YM-coupling limit of the
theory expanded around its BPS instantonic configurations, via the
identification of the interacting string diagram with the spectral curve of the
relevant configuration. Both the GS action and the perturbative weight
$g_s^{-\chi}$, where $\chi$ is the Euler characteristic of the world-sheet
surface and $g_s$ the string coupling, are obtained.
