The individual differences in DNA (deoxyribonucleic acid) are used to study the genes and their behaviour.
The gene expression levels are partly influenced by the environment (epigenetic factors). Different environmental factors such as drugs, culture, trauma, chronic stress, etc. influence the gene expression levels. In this, the gene sequence is not altered, but the expression levels are altered. Thus, differences in gene expression levels are considered in the study of epigenetics.
Thus, the blanks can be filled with DNA; gene expression.