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Regent Theatre marketing manager Hannah Molloy said theatre director Sarah Anderson decided to install a new sign on the toilet door, near the bar downstairs.
The sign was "relevant and useful'' as it recognised the subject of gender identity in public spaces, Ms Molloy said.
The sign included internationally recognised symbols for toilets for females, males, non-gender specific people and disabled people.
Below the symbols was a reclining alien and the words "Whatever, just please wash your hands''.
The reclining "little green friend'' was relaxed, Ms Molloy said.
"We like people to be relaxed and that's the point of having the sign. It takes any anxiety away for people who might feel pressured into going into toilets specified for one gender, or another, when they don't necessarily relate to that gender.''
The sign gave people another option, allowing them "to feel more comfortable and safe''.
Feedback on the sign since its installation on Friday had been positive, she said.
"People are delighted ... we haven't had any negative feedback at all. Clearly there is a demand for it - society is ready.''