show subcategories only of a selected parent category

hi,
I’ve searched for hours but I can’t find any solution.

What I’m looking for is how to display categories and subcategories of a selected one.
here is an example
Suppose my post is in “subx cat1” I’d like to show as below:

cat1
-subx cat1
-suby cat1
-subz cat1
cat2
cat3
cat4

all categories have subones.. but I don’t want to show them all.
Right now my code is showing all:

<?php
 $catsy = get_the_category();
$myCat = $catsy[0]->cat_ID;
$categories = wp_list_categories('show_count=0&exclude=1&title_li=<h2>MY TITLE</h2>&echo=0&hide_empty=0&'.$currentcategory);
$category_array = preg_split('/\n/', $categories);
$count = count($category_array);
$i = 0;
while ( $i < $count ) {
        if ( preg_match('/<ul class=(\'|")children(\'|")/i', $category_array[$i+1]) ) {
                echo preg_replace('/<li class=(\'|")(.+)(\'|")>/i', '<li class=$1parent $2$3>', $category_array[$i]) . "\n";
        } else {
                echo $category_array[$i] . "\n";
        }
        $i++;
}
?>

any help?
thanks