Here's why:

* DNA ligase's function: It's an enzyme that joins together (ligates) two strands of DNA, which is a fundamental process in DNA replication, repair, and genetic recombination. These processes are central to molecular biology.

* Focus of molecular biology: Molecular biology focuses on the structure and function of biological molecules, especially nucleic acids (DNA and RNA) and proteins.

* Other fields involved: While DNA ligase is primarily studied in molecular biology, its study also involves aspects of biochemistry (enzymes and their mechanisms) and genetics (DNA's role in inheritance).

So, while DNA ligase's study overlaps with other fields, its core function and study place it firmly within the realm of molecular biology.