setTextformat and defaultTextformat

Hi,

Can someone explain the difference? Here is my problem

I set a textformat and apply it with setTextFormat, this is fine when my movie runs however when I try and change the text (with a mouse over of a button) this formatting disappears and have to set it again after I have changed the value of the text box. Why does it resort back to the original formatting? Is there not a way to set a textfield and keep it set? I know it is just one line but it is not ideal.

Thanks,

function setText(evt:MouseEvent) {
clientText.text = “hello2”;
clientText.setTextFormat(myFormat);

}