An example of the code to place in a custom module for limiting access to a specific content type, in this case the "custom content type" content type.

hook_node_access_example.php

<?php
/**
 * Implement hook_node_access().
 */
// Add perms to the custom_content_type content type

function HOOK_node_access($node, $op, $account) {
  $type = is_string($node) ? $node : $node->type;

  if ($type == 'custom_content_type' && $op == 'view') {
    // Define roles that can see custom_content_type content type content.
   global $user;
   $custom_content_type_perms = array_intersect(
       array(
     'role1',
     'role2',
     'role3'
     ),
       array_values($user->roles));
   // If they have any of these roles, then they can see the custom_content_type content
   if ($custom_content_type_perms){
      return NODE_ACCESS_ALLOW;
    }
    else {
      return NODE_ACCESS_DENY;
    }
  }
  return NODE_ACCESS_IGNORE;
}