The S&P/ASX 200 Index (ASX: XJO) is having a poor session on Thursday. In afternoon trade, the benchmark index is down 0.5% to 7,960.9 points.

Four ASX shares that are falling more than most today are listed below. Here's why they are dropping:

Domain Holdings Australia Ltd (ASX: DHG)

The Domain share price is down almost 5% to $4.26. This morning, the property listings company revealed that CoStar Group, Inc. (NASDAQ: CSGP) has improved its takeover offer. Domain revealed that CoStar has lifted its offer from $4.20 cash per share to $4.43 cash per share. However, this is lower than yesterday's close price of $4.47, which appears to indicate that the market was expecting a much better offer. The Domain board has unanimously determined to engage with CoStar to facilitate due diligence.

Mesoblast Ltd (ASX: MSB)

The Mesoblast share price is down almost 3% to $2.14. This is despite the allogeneic cellular medicine developer making a positive announcement this morning. Mesoblast revealed that its Ryoncil (remestemcel-L) product has gone on sale in the United States. The release notes that Ryoncil is the first mesenchymal stromal cell (MSC) therapy approved by U.S. Food and Drug Administration (FDA) for any indication. It is now available for approved for treatment of paediatric patients two months and older with steroid-refractory acute graft versus host disease.

Pro Medicus Limited (ASX: PME)

The Pro Medicus share price is down 7% to $211.06. This has been driven by a selloff of tech stocks on Thursday following a poor night on Wall Street. In addition, this morning Macquarie retained its neutral rating on the health imaging technology company's shares but with a slightly trimmed price target of $257.40. Though, it is worth noting that this still implies potential upside of approximately 22% for investors over the next 12 months. Not bad for a neutral rating.

Tuas Ltd (ASX: TUA)

The Tuas share price is down 9% to $5.27. This Singapore based telco company's shares have been under pressure this week after it released its half year results. Tuas reported revenue of $73.2 million and a maiden net profit after tax of $3 million. While both were up sharply on the prior corresponding period, that hasn't stopped investors from hitting the sell button. The team at Citi believes this is a buying opportunity for investors. This morning, the broker retained its buy rating and $7.10 price target. This suggests that upside of almost 35% is possible from current levels.