List Indexes - Short Function Without Options?

I have 2 lists and function to clean them
function del_sel(option){
if (option == "1"){
document.forms[0].list1.innerHTML = '';
}if (option == "2"){
document.forms[0].list2.innerHTML = '';
}}

How to make this function shorter & without options? Something like that:
function del_sel(option){
document.forms[0].option.innerHTML = '';
}

The problem is that this code returns mistake:
function del_sel(option){
document.forms[0].option.innerHTML = '';
}
...
<input type='button' value='Delete onclick='del_sel(form.list1);>