ESR1 and cancer: In addition, personalized cancer treatments require a complex invasive and time-consuming analysis of many different parameters, such as histological type and grading, evaluation of Estrogen Receptor (ER), Progesterone Receptor (PgR), HER2 and Ki67 protein expression by immunohistochemistry (IHC), gene mutation analysis by DNA sequencing and chromosomal alterations by Fluorescence In Situ Hybridization (FISH)3.