Tropogen

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The protein on the outer surface of a virus that combines with the cellular receptor for the virus. The interaction between the viral tropogen and the cellular receptor is quite specific. This interaction may play a role in determining tissue tropism and host range.

An example of a tropogen is gp120, an envelope glycoprotein of HIV. gp120 combines in a specific fashion to its cellular receptor CD4, a protein found on various cells in the human immune system including T-helper cells, and macrophage.