The S&P/ASX 200 (Index:^AXJO) (ASX:XJO) fell heavily into negative territory at the start of the day as markets continue to be spooked by Brexit.

Investors have since been lured back in and the main index is now only 0.45% lower to 5,113 points.

Despite the mid-afternoon turnaround, these four shares have continued to suffer steep declines:

Sydney Airport Holdings Ltd (ASX: SYD)

Sydney Airport shares have fallen more than 4% today and are now trading at $6.98. This is surprising considering the stock is generally viewed as one of the best defensive shares on the ASX and has been one of the best performing shares over the past five years. A number of factors have impacted the shares today, however, including a broker downgrade from Credit Suisse and concerns that the fall in the British pound against the Australian dollar could potentially result in fewer tourists from the UK. Sydney Airport shares are also likely to be the subject of profit taking after a very strong performance over the past 12 months.

Shares of Sydney Airport have gained 40% over the past 12 months.

Platinum Asset Management Limited (ASX: PTM)

Platinum Asset Management is one of the worst-performing fund managers today with its shares falling by 5.5% to $5.50. Although it has a smaller exposure to the UK and Europe compared to BT Investment Management Ltd  (ASX: BTT) and Henderson Group plc (ASX: HGG), it nevertheless is highly exposed to international equity markets that are volatile at the moment. Investors will also be concerned about the ability of fund managers like Platinum to attract new fund inflows while markets remain volatile.

Platinum Asset Management shares have lost 28% over the past 12 months.

Aconex Ltd (ASX: ACX)

Aconex shares have plunged nearly 5% today in what appears to be a case of profit taking amid investor concerns about its exposure to the UK and European region. The tech disruptor has been one of the top performing stocks over the past 12 months so the volatility and uncertainty caused by the Brexit vote will have given some investors a reason to lock in profits. Aconex is also the dominant provider of online construction project management in the UK and Europe and any instability in the region could be viewed as a risk to the region’s construction industry.

Despite recent weakness, Aconex shares have still managed to gain 109% over the past 12 months.

Santos Ltd (ASX: STO)

Santos is one of the worst performing energy stocks today with its shares falling around 3.4% to $4.50. The sector overall has lost around 2% today thanks to another decline in crude oil prices overnight. The Brexit decision has put the oil price under major pressure with investors worrying that a slowdown in global economic growth could reduce the demand for the commodity. Investors should note that Santos’ share price is one of the most volatile because the company remains highly leveraged and operates on smaller margins than some of the other major producers on the ASX.

Santos shares have fallen more than 45% over the past 12 months.

I wouldn't be a buyer of Santos shares at the moment and there are a number of stocks you might want to avoid as well...

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Motley Fool contributor Christopher Georges owns shares of BT Investment Management Limited and Platinum Investment Management Limited. 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.