Ethnicity and genetics are partly responsible for "heterogeneity", or the genetic differences from one area of tissue to another. These differences may show up as different types of cells or mutations of the cells themselves.
For this research, scientists wanted to explore heterogeneity in normal breast tissue, breast cancer tissue and tissue adjacent to breast cancer tumors. By looking at ethnicity and genetics, they hoped to identify markers that could shed light on why women of different ethnicities or genetic predispositions develop certain types of breast cancer.