Why do you need to attach it to the HTML? Just bind the function with hover
$("div.system_box").hover(function(){ mousin },
function() { mouseout });
If you do insist to have JS references inside the html, which is usualy a bad idea you can use:
onmouseover="yourJavaScriptCode()"
...
$("div.system_box").click(function(){ slideonlyone($(this).attr("data-target")); });