HTML Multiple select

Hoe do I insert into a table for a single field a html multiple select drop down list.

I have this to create my drop down list.

# create SQL statement
$sql = "SELECT * FROM genre
ORDER BY gid ASC";

# execute SQL query and get result
$sql_result = mysql_query($sql,$connection)
or die("Couldn't execute query.");

# put data into drop-down list box
while ($row = mysql_fetch_array($sql_result)) {

$os = $row["genre"];
$option_block .= "<OPTION value="$os">$os</OPTION>";
}

And this to insert into my table "address"

if ($REQUEST_METHOD=="POST") {

# double-up apostrophes
$description = str_replace("'","''",$description);
$sitename = str_replace("'","''",$sitename);

# setup SQL statement
$SQL = " INSERT INTO address ";
$SQL = $SQL . " (address, city, state, zip, phone, email, fname, lname, country, contact, instr) VALUES ";
$SQL = $SQL . " ('$address', '$city', '$state', '$zip', '$phone', '$email', '$fname', '$lname', '$country', '$contact', '$instr') ";

#execute SQL statement
$result = mysql_db_query($db,"$SQL",$connection);

# check for error
if (!$result) { echo("ERROR: " . mysql_error() . "n$SQLn"); }

echo ("<P><B>New Member Added</B></P>n");

}

But I need the field $instr to be for the multiple insert. I'm lost. Please help