The S&P/ASX 200 (Index: ^AXJO) (ASX: XJO) has posted another down day, closing down 0.7%, but it could’ve been much worse if we’d followed the lead of US markets. On Friday, the Dow Jones plunged 2.4%, while the broader S&P 500 was heavily sold off – losing 2.2%.

That makes it 10 down days in 11 trading sessions. Year-to-date the ASX 200 is down over 8%, similar to US markets.

These four companies fell even more than the index…

Santos Ltd (ASX: STO) saw its share price plunge 8.4% to $2.63 as oil prices took another hit, with the benchmark Brent Crude falling 6.3% to US$28.94 a barrel. The future doesn’t look any brighter either, with Bloomberg reporting that Iran is expected to boost exports of oil now that sanctions have been lifted – adding even further to the global oversupply and oil prices at 12-year lows.

Mesoblast limited (ASX: MSB) saw its share price sink 7.1% to $1.51, and shares have now lost 62% of their value in the past 12 months. A highly-rated biotech company amongst institutional investors, Mesoblast has a number of stem cell products it wants to bring to market, including 5 in Phase 3 trials. Mesoblast listed on the US NASDAQ exchange late last year at a substantial discount to the equivalent trading price on the ASX, and it’s been all downhill since.

The Magellan Financial Group Ltd (ASX: MFG) share price dropped 6.3% to $23.35 as investors bet that equity market falls will take a toll on the fund manager’s funds under management (FUM). Most fund managers charge a percentage of FUM as their fee, so lower FUM means lower revenues. However, Magellan’s share price has now lost more than 16% year-to-date – double the losses on US and Australian share markets – suggesting it may have been oversold.

Adacel Technologies Limited (ASX: ADA) share price sank 7.6% to $2.08. One of 2015’s hot companies, with the share price rising more than 700%, Adacel provides traffic control software and  simulators. The company has forecast a 50% increase in revenues in the 2016 financial year for a number of reasons – which we outlined here. Perhaps Adacel shares have soared too high, and some investors have decided to take some profits.

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Motley Fool writer/analyst Mike King doesn't own shares in any companies mentioned. You can follow Mike on Twitter @TMFKinga

Unless otherwise noted, the author does not have a position in any stocks mentioned by the author in the comments below. 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.