Chemicals or finished products that cause a classical allergic response are defined as skin sensitizers. The key mechanisms leading to the allergic response have been defined and involve four key events which are as follows: 1) Covalent binding electrophilic substances to nucleophilic centers of skin proteins, 2) Induction of an inflammatory response in the keratinocyte, 3) Activation of dendritic cells and the 4) T-cell proliferation. A panel of in vitro tests can be used that focus on these key events. The in vitro methods for In Vitro skin sensitization have been adopted by and described in the OECD Test Guideline No. 442E. OECD test guideline 442D describes an in vitro test the focused on the second key event which occurs in keratinocytes and includes inflammatory responses and changes in gene expression associated with the antioxidant/electrophile response element (ARE). OECD test guideline No. 442C focuses on the first event in the induction of dermal sensitization by chemicals. These tests may be selected and run individually or as a panel of tests in a decision tree.