Hi Shakuntala,
There are two ways you can do this.
From the homepage when you run a query, there's an option to 'Select Patient/Case Set'. Here you can select Breast Cancer from the dropdown menu:
Once you select this, your query will only run in Breast Cancer samples.
Alternatively, if you want a subset of samples which is not available in that list, you can first explore a study in study view. There, you can apply filters and then use the query box in the upper right to enter your gene of interest and run a query in just those samples you've filtered to:
I hope this helps!
-Tali