This professional-only repair treatment is the second step of the revolutionary Olaplex Original Bond Builders system offered as a single step so you can restock at your own pace. While Nº.1 (sold separately) is added to color, lightener, and chemical services, Nº.2 is the post-color/chemical service step to target bonds that may have been missed by Nº.1. It rebuilds all three bonds, including disulfide bonds, to leave hair stronger, softer, more manageable hair in just 3 minutes at the bowl before shampooing.