Gabe Newell met with a group of deaf gamers to get a better idea of what Valve could do to make their games more accessible for them. Newell also mentioned possibly incorporating sign language into Half Life 2: Episode 3.

A lot of good points were brought up in the meeting, like how stronger facial expressions for characters could help convey their emotions to people who cannot hear the tone of their voice, or how there is a difference between how a deaf person uses sign language and how a hearing person does.

Gabe Newell told the group that he had the idea that Alyx could have previously had a crush on a deaf person and taught Dog how to sign so she could practice. He said Alyx and Dog might use sign language to communicate when they want to stay quiet or don’t want others to understand them.

[Source: The Escapist]