The small part of the nucleus where ribosomal subunit formation takes place is called the nucleolus.

Here's a breakdown:

* Nucleus: The control center of the cell, containing the genetic material (DNA).

* Nucleolus: A dense region within the nucleus, not bound by a membrane. It's the site of ribosome biogenesis.

* Ribosomal Subunits: The two parts of a ribosome, the small subunit and the large subunit, which come together to translate mRNA into proteins.

The nucleolus is essentially a ribosomal factory. It synthesizes ribosomal RNA (rRNA) and assembles it with ribosomal proteins to create the ribosomal subunits. These subunits then exit the nucleus through nuclear pores and join to form functional ribosomes in the cytoplasm, where they play a crucial role in protein synthesis.