Use Of Speech Marks When Writing Function To HTML

I've been using the document.write function to loop through some xml and print the data within a HTMl table. I thought it would be useful to add a hyperlink to the table row, so I thought the best way to do this would to create a function (called popup), which takes an id as a parameter.

The problem with this, is that when writeing using the onClick, the speech marks seem to all get messed up.

Code:

onclick='popup('"+mailID+"')'

I'm 99% sure it's just the speech marks, i'm just not sure of the right combination, and it's doing my head in

If someone could even point me in the right direction, even the proper name as to what this is called would be great, as googleing it is proving to be a nightmare!

Here's the whole section of code if it's more useful:

Code:

var mailID = x[i].getElementsByTagName("mailid")[0].childNodes[0].nodeValue;
if (r == "unread") { //email is unread
document.write("<tr onclick='popup('"+mailID+"')' title='test'>"); } else { //else is read, so add colour

[Code]....