function bp_get_the_notification_mark_unread_link( $user_id = 0 ) {
		// Set default user ID to use.
		$user_id = 0 === $user_id ? bp_displayed_user_id() : $user_id;

		$retval = sprintf( '<a href="%1$s" class="mark-unread primary bp-tooltip">%2$s</a>', esc_url( bp_get_the_notification_mark_unread_url( $user_id ) ), __( 'Unread', 'buddypress' ) );

		/**
		 * Filters the link used for marking a single notification as unread.
		 *
		 * @since 1.9.0
		 * @since 2.6.0 Added $user_id as a parameter.
		 *
		 * @param string $retval  HTML for the mark unread link for the current notification.
		 * @param int    $user_id The user ID.
		 */
		return apply_filters( 'bp_get_the_notification_mark_unread_link', $retval, $user_id );
	}