The aim of this article is twofold: firstly, to describe the seven-year analytic treatment of a TG adolescent (F “April” to M “Tran”) and, secondly, based on the clinical observations, to propose a reflection on the intrapsychic events linked to gender transition. We could witness during this analysis that the dissonant anatomical sex, which is at the heart of the gender dysphoria, resists mentalization and consequently its psychological integration. The psychic events of transition, understood here on the model of a mourning process, could denote the various strategies necessary to the TG individual to negotiate the obstacle of mentalization.