Quantum Computing (QUBT) had the best month in 2013 by a mid-cap US stock in February 2013, returning 566.7%.

ASSET	MONTH	% RETURN
Quantum Computing (QUBT)	February 2013	566.67%
Quantum Computing (QUBT)	April 2013	242.86%
Centrus Energy (LEU)	July 2013	174.48%
Plug Power (PLUG)	May 2013	137.5%
Camtek (CAMT)	November 2013	125.82%
Plug Power (PLUG)	December 2013	106.67%
Perdoceo Education (PRDO)	October 2013	100.74%
Applied Digital (APLD)	February 2013	92%
Arrowhead Pharmaceuticals (ARWR)	July 2013	85.71%
ArcBest (ARCB)	May 2013	82.89%
Cytokinetics (CYTK)	January 2013	76.12%
Globalstar, Inc Common Stock (GSAT)	January 2013	71.88%
Globalstar, Inc Common Stock (GSAT)	September 2013	67.69%
JOYY (JOYY)	May 2013	63.77%
ACADIA Pharmaceuticals (ACAD)	April 2013	62.55%
MGIC Investment (MTG)	March 2013	62.3%
Cytokinetics (CYTK)	June 2013	62.04%
JOYY (JOYY)	July 2013	58.93%
Ambarella (AMBA)	March 2013	58.82%
Globalstar, Inc Common Stock (GSAT)	May 2013	54.29%
Catalyst Pharmaceuticals (CPRX)	April 2013	54.08%
Catalyst Pharmaceuticals (CPRX)	September 2013	53.27%
Addus HomeCare (ADUS)	May 2013	52.2%
Addus HomeCare (ADUS)	March 2013	51.61%
Corcept Therapeutics (CORT)	December 2013	48.61%

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