Trying to customize NGUI - UIButton in the Inspector

Question

When I try to customize a class of "NGUI - UIButton", I got a problem that's not working serialize variables in Unity editor inspector view.

public class MyButton : UIButton // inherited NGUIButton class
{
    public bool _isChecked;
    public int _count;
} 

I can't see the variables in the inspector of Unity editor.

How can I see the variables in inspector?

I was trying it with [serializeField] script too. But it's also not working

Is there something special about NGUI?