Dear Sir/Madam

A quick question.

I have 11 industries (i1 - i11) and I want to run a regression that only contains i1, i5, i6 (for example).

Is there any code for this?

I have tried
Code:
xtreg Y A B C D if i1 i5 i6
which doesn't work;
Code:
]xtreg Y A B C D if industry == i1 &  i5 & i6
which doesn't work as well;

Really appreciate if you could offer any help.

Neng