A lesion similar to the one shown in Figure 2 but it has advanced further into the enamel which is now more porous. The surface remains smooth and not cavitated but the enamel is now quite porous. The dentine is now involved because acid can penetrate the full distance. The response from the dentine is mineralisation of the lateral canals to improve the defences to acid penetration leading to the formation of the so-called “translucent zone”. Note the development of a translucent zone from both the proximal lesion as well as the occlusal fissure.