I'm not sure what you mean by "the Expression field".
But anyway, in order to give the best answer, I think we need to know a
little more. In particular, where is this checkbox? If on a form, is it a
single view form or a continuous view form or something else? And is it
bound to a field in a table? If so, what is the name of the field and what
is the name of the table? Thanks.