Pre_get_posts

functions.php
function my_home_category( $query ) {
if ( $query->is_home() && $query->is_main_query() ) {
$query->set( ‘cat’, ‘1’ );
}
}
add_action( ‘pre_get_posts’, ‘my_home_category’ );

I’m trying to set up to appear more than one category entretando still fails.
I tried:
$ query-> set (‘cat’, ‘1, 2 ‘);
or
$ query-> set (‘cat’, array (‘1 ‘, ‘2’)));

but it still fails