A proposed model to merge the dual roles of PTBP1 and PTBP2 in neuronal differentiation and cryptic exon repression. Initially, levels of PTBP1 are high in undifferentiated cells. During neuronal differentiation, PTBP1 expression decreases and PTBP2 is increased. However, while the increase in PTBP2 expression is sufficient to repress nonconserved cryptic exons, it is insufficient to repress conserved neuronal exons. Thus, in mature neurons, total repression levels are reduced to activate exons that are important for neuronal differentiation, but not reduced enough to allow the incorporation of deleterious nonconserved cryptic exons.