SAN FRANCISCO – Investor confidence in Exact Sciences Corp. has started to stumble in recent months. The Madison, Wis.-based company’s valuation peaked at about $15.5 billion just a few short weeks after it announced that it would acquire Genomic Health Inc. in late July. Its market cap trajectory has been uneven since and took another hit when the company preannounced 2019 earnings at the J.P. Morgan (JPM) Healthcare Conference, falling to around $13.5 billion.