Summit Therapeutics (SMMT) had the lowest return in June 2018 by a large-cap US stock, returning -81.5%.

ASSET	MONTH	% RETURN
Summit Therapeutics (SMMT)	June 2018	-81.54%
YPF Sociedad Anonima (YPF)	June 2018	-25.75%
BeiGene (ONC)	June 2018	-23.14%
First Solar (FSLR)	June 2018	-22.63%
Centrais Elétricas Brasileiras SA - Eletrobrás (EBR.B)	June 2018	-22.17%
Centrais Electricas Brasileiras (EBR)	June 2018	-20.15%
Petroleo Brasileiro Petrobras SA ADR (PBR)	June 2018	-17.25%
Stellantis (STLA)	June 2018	-17.15%
Banco Santander Brasil SA ADR (BSBR)	June 2018	-17.13%
Insmed (INSM)	June 2018	-15.72%
Petróleo Brasileiro SA - Petrobras (PBR.A)	June 2018	-15.65%
Penumbra (PEN)	June 2018	-14.88%
Starbucks (SBUX)	June 2018	-14.06%
Banco Bradesco SA ADR (BBD)	June 2018	-13.54%
argenx NV ADR (ARGX)	June 2018	-13.33%
Wynn Resorts (WYNN)	June 2018	-13.3%
Brown-Forman (BF.B)	June 2018	-13.29%
Allegheny Technologies (ATI)	June 2018	-13.2%
Companhia de Saneamento Basico do Estado de Sao Paulo SABESP ADR (SBS)	June 2018	-13.15%
Lam Research (LRCX)	June 2018	-13.11%
Stifel Financial (SF)	June 2018	-12.81%
ON Semiconductor (ON)	June 2018	-12.74%
Carpenter Technology (CRS)	June 2018	-12.69%
Interactive Brokers (IBKR)	June 2018	-12.37%
ArcelorMittal SA ADR (MT)	June 2018	-12.36%

Large-cap stocks are treated as having a market cap of $10B+.