Billionaire industrialist Charles Koch on Tuesday announced his brother David Koch is stepping away from a day-to-day role with the family’s business for health reasons and is expected to take similar steps to unwind his involvement with the advocacy machine the Kochs have used to shape American politics. Charles Koch made the announcement via an email message sent to Koch Industries employees and obtained by TIME. “We are deeply saddened by this, as we miss David’s insightful questions and his many contributions to Koch Industries,” Charles Koch wrote.