Synthetic spectra of a M7 PMS star (solid line) as provided by Allard et al. (2012), together with the observed spectra of two young M7 stars (in dots and dashes, Muench et al. 2007). The blue, green and red lines stand for the transmission curves of the 2MASS JHK_S filters respectively. We find good agreement between the model and the observations in the JK_S bands, while in the H band the model overestimates the radiated flux (or underestimates the opacity).