Adding multiple conditions for custom post types

I’m kinda stumped on this one..

I’m using custom post types on my website, and for one of my custom post types I don’t want an author box to display. My custom post type functions like a portfolio. Basically what I want is that the author box show up on all is_single posts except for the portfolio is_single posts. Here is what I have in my functions.php file:

function author_box() {
if (is_single() && post_type_exists('folio')) {
}
elseif (is_single()) { ?>

yada yada yada

<?php }
}

This isn’t working from what I can tell, so how I can I accomplish this with the conditional tags?

Thomas