Selectivity index (SI). SI is an index of selectivity more frequently used. To be measured for a single peptide following the equation (Eq.3), the index requires both IC50 values against a normal healthy cell line (HCL) and a cancer cell line (CCL). According to Badisa and co-workers , a higher SI value means greater selectivity, while a value less than 2 indicates the general toxicity of a peptide. SI = IC50 HCL / IC50 CCL (Eq.3)