Skin rash is the principal toxicity of both small molecule and antibody therapeutics targeting the epidermal growth factor receptor (EGFR), and retrospective analyses of a wide variety of studies have suggested that skin toxicity correlates with clinical benefit (Cohen et al, 2003; Perez-Soler et al, 2004; Saltz et al, 2004; Shepherd et al, 2005; Gibson et al, 2006; Jonker et al, 2007; Van Cutsem, 2007; Van Cutsem et al, 2007; Wacker et al, 2007). This evidence concerns the gene EGFR and exanthem.