**Core Concept**
Measles, Mumps, and Rubella (MMR) vaccine is a live attenuated vaccine, expressed in terms of the number of cell cultures, which is a measure of its potency. The MMR vaccine is a combination vaccine that protects against three viral infections: measles, mumps, and rubella.

**Why the Correct Answer is Right**
The MMR vaccine is expressed in terms of the number of cell cultures, which is a measure of its potency. The number of cell cultures is a way to standardize the vaccine's strength and ensure that it contains a consistent amount of the live attenuated viruses. This is similar to how vaccines like MMR are expressed in terms of the number of doses or units.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because MMR vaccine is not expressed in terms of the number of doses or units. While the number of doses may be relevant for some vaccines, it is not a standard way to express the potency of the MMR vaccine.
**Option B:** This option is incorrect because the MMR vaccine is not expressed in terms of the number of antigens or antibodies. While antigens and antibodies are important components of the immune response, they are not used to express the potency of the MMR vaccine.
**Option C:** This option is incorrect because the MMR vaccine is not expressed in terms of the number of viral particles or genome copies. While the number of viral particles or genome copies may be relevant for some vaccines, it is not a standard way to express the potency of the MMR vaccine.

**Clinical Pearl / High-Yield Fact**
Live attenuated vaccines like MMR are typically expressed in terms of the number of cell cultures, which is a measure of their potency. This is an important consideration for healthcare providers when administering vaccines to patients.

**Correct Answer: C. The number of cell cultures.**