Command to Clear a Combo Box in Excel

Hi,
Can anyone help me with a simple command line in Excel?
I have a couple of combo boxes that pull values from a table in another worksheet. I made a button that will go through all the fields in my workbook and set the default values to it. All vb codes work fine for all the cells, but I don't know the command to set the value in a combo box. I looked everywhere, but couldn't find any answer. What I need is: when the "DEFAULT" button is pushed, the targetted combo box(es) will display Nothing or a value of a cell in another worksheet. Any help is appreciated.