Having trouble with join

I have a subquery that I'd like to make a join instead to see if it is
any faster.

subquery:  select distinct cell from center_out_cell where rep in
(select rep from center_out_opto where marker = 5);

All this does is find the occurrences of 'cell' that have 'marker' = 5.
However, 'cell' is defined in table center_out_cell and 'marker' is
defined in table 'center_out_opto'. Both tables have 'rep' so that ties
them together. The subquery gives me the correct results.

I tried this for a join but didn't get the right result:

select distinct cc.cell from center_out_cell cc, center_out_opto co
where co.marker = 5 and co.rep = cc.rep;

Can anyone tell me what I'm doing wrong?
Does this sound more like a subquery or a join?

Thanks
-Tony
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