The stock prices for 14 companies in the 360Dx Index were down during the month, while 13 firms' share prices fell.
In its response, the company accused the Securities and Exchange Commission's lawsuit of containing "serious factual inaccuracies."
CareDx and Invitae each had massive share-price gains last month. Exact Sciences, GenMark, and Natera also saw sizeable increases in their stock prices.
Proceeds from the offering, expected to close on or about Aug. 14, could be used to fund acquisitions, to repay existing debts, or for working capital.
Natera, Invitae, and Opko Health were the biggest gainers in the 360Dx Index last month, while Enzo Biochem had the steepest decline.
Month over month, the 360Dx Index beat the Dow Jones Industrial Average, the Nasdaq Composite, and the Nasdaq Biotech Index.
The study solidified how doctors should interpret and act upon an intermediate result from the breast cancer risk test in how to treat early-stage patients.
Overall, 20 of the 25 firms in the index saw their share prices grow in May, including seven whose stocks rose more than 20 percent compared to April.
The investment bank cited disappointing first quarter financial results, a slow uptake to the business, and competitive pressures for the downgrade.
The share prices of 20 companies in the 360Dx Index rose last month, with Exact Sciences, Natera, Invitae, and NeoGenomics the biggest gainers.