Protagonist Therapeutics (PTGX) had the highest return in May 2020 by a mid-cap US stock, returning 148.6%.

ASSET	MONTH	% RETURN
Protagonist Therapeutics (PTGX)	May 2020	148.57%
Fastly, Inc Class A Common Stock (FSLY)	May 2020	105.23%
Hut 8 Corp Common Stock (HUT)	May 2020	100%
Liquidia Technologies (LQDA)	May 2020	89.34%
United Natural Foods (UNFI)	May 2020	85.26%
Riot Blockchain (RIOT)	May 2020	81.36%
Caesars Entertainment (CZR)	May 2020	79.82%
Ideaya Biosciences (IDYA)	May 2020	73.89%
American Axle & Manufacturing (AXL)	May 2020	73.41%
Viridian Therapeutics (VRDN)	May 2020	73.13%
Wix (WIX)	May 2020	71.31%
Celcuity LLC (CELC)	May 2020	61.9%
Beam Therapeutics (BEAM)	May 2020	61.61%
TG Therapeutics (TGTX)	May 2020	61.19%
RH (RH)	May 2020	56.06%
Fortuna Silver Mines (FSM)	May 2020	55.36%
Acm Research (ACMR)	May 2020	55.3%
Tutor Perini (TPC)	May 2020	54.19%
New Gold (NGD)	May 2020	53.16%
USA Compression Partners (USAC)	May 2020	49.88%
Appfolio (APPF)	May 2020	49.75%
Century Communities (CCS)	May 2020	46.02%
Resideo Technologies (REZI)	May 2020	42.91%
Genesis Energy (GEL)	May 2020	42.45%
Wayfair (W)	May 2020	42.16%

Mid-cap stocks are treated as having a market cap between $2B and $10B.