Just when it looked like the S&P/ASX 200 (Index: ^AXJO) (ASX: XJO) might finally climb above its pre-Brexit trading level, the index has tumbled back down by around 1% to 5,230 points today.

The banks have once again acted as a drag on the market, with the big four all down over 1% so far. Going against today’s downtrend have been four shares in particular.

Each have posted strong gains, much to the delight of their respective shareholders. Here’s why:

Catapult Group International Ltd (ASX: CAT) shares are up by over 5% to $3.32 today following yet another positive trading update from this growing company. The leading sports analytics company announced that it has seen four consecutive quarters of record sales. This led to 8,354 unit sales for a total contract value of $29.4 million in FY 2016, well ahead of its forecast of 8,000 units and $24.5 million.

Catapult Group International shares are now up by an incredible 209% in the last 12 months.

Jumbo Interactive Ltd (ASX: JIN) shares are up a massive 20% to $1.57 after the interactive lottery business released its full year guidance to the market. The company revealed that it expects net profit after tax of $4.4 million, which is an increase of 528% year on year. Whilst things do look very promising for the company, its lack of a long-term agreement with lottery operator Tatts Group Limited (ASX: TTS) is something that concerns me.

Jumbo Interactive shares are now higher by almost 50% in 2016.

Pro Medicus Limited (ASX: PME) shares have soared for a second day running, this time by over 7% to $5.50. The company appears to have come onto investors’ radars since winning a key new $18 million contract for its popular Visage 7 technology. This is the third key contract win in the United States in as many months for the medical imaging company. Whilst the shares are changing hands at approximately 100x annualised earnings, its exciting growth prospects may just justify the premium.

Pro Medicus shareholders are sitting on a gain of 154% in the last 12 months.

Smartgroup Corporation Ltd (ASX: SIQ) shares have jumped almost 7% to $6.19 today following an announcement that the salary packaging specialist had entered into an agreement to acquire Autopia for $36 million. Investors appear to be bullish on the deal which management estimates will be accretive to earnings by 7% in the 2016 calendar year. Autopia is a novated leasing company in the corporate market and currently manages around 3,000 vehicles.

Smartgroup shares have jumped by around 28% in the last three months.

Finally, if you're tempted to add in one of these shares to your portfolio then I would recommend making space by removing one of these three rotten ASX shares if you own them. Each one of them could potentially be doing more harm than good to your portfolio.

3 Rotten Shares to Sell, and 1 to Buy Today

After a double-digit rally for the ASX since 2016 lows, investors should be on high alert. You'll find a full rundown below of 3 shares we think you should avoid today plus one top pick worth buying, even if the market turns south and the RBA keeps rates at an "emergency low." Simply click here to uncover these stocks.

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Motley Fool contributor James Mickleboro has no position in any stocks mentioned. The Motley Fool Australia has no position in any of the stocks mentioned. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy. This article contains general investment advice only (under AFSL 400691). Authorised by Bruce Jackson.