PTC Therapeutics (PTCT) had the highest return in January 2014 by a mid-cap US stock, returning 55.8%.

ASSET	MONTH	% RETURN
PTC Therapeutics (PTCT)	January 2014	55.83%
TG Therapeutics (TGTX)	January 2014	51.52%
Travere Therapeutics (TVTX)	January 2014	47.15%
Plug Power (PLUG)	January 2014	46.6%
MakeMyTrip (MMYT)	January 2014	43.41%
Arrowhead Pharmaceuticals (ARWR)	January 2014	42.7%
Energy Fuels (UUUU)	January 2014	36.74%
American Airlines (AAL)	January 2014	33.83%
Supernus Pharmaceuticals (SUPN)	January 2014	30.09%
Skyline (SKY)	January 2014	28.21%
Vipshop (VIPS)	January 2014	27.2%
BlackBerry (BB)	January 2014	26.85%
Enpro Industries (NPO)	January 2014	25.81%
Qualys (QLYS)	January 2014	25.45%
Camtek (CAMT)	January 2014	24.64%
Patrick Industries (PATK)	January 2014	24.52%
Perpetua Resources (PPTA)	January 2014	24.29%
Under Armour Inc A (UAA)	January 2014	23.84%
Fortuna Silver Mines (FSM)	January 2014	22.71%
Alkermes (ALKS)	January 2014	20.38%
Advanced Energy Industries (AEIS)	January 2014	20.11%
Applied Digital (APLD)	January 2014	20%
Cytokinetics (CYTK)	January 2014	19.78%
Jazz Pharmaceuticals (JAZZ)	January 2014	19.65%
Silicon Motion Technology (SIMO)	January 2014	18.94%

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