How One Major Healthcare Investor Let a $185 Million Biotech Position Grow Despite Trimming Its Stakes

The Paradox That Tells an Investor Story

When you reduce your holdings in a stock, you’d typically expect the position value to shrink. But that’s not what happened with Eventide Asset Management’s biotech bet on Mirum—a move that reveals something important about long-term conviction in an emerging growth company.

On November 14, the Boston-based newspaper holder and active asset manager disclosed that it had pared back its Mirum Pharmaceuticals (NASDAQ:MIRM) position by 114,922 shares during the third quarter. Yet instead of shrinking, the overall value of that holding ballooned by roughly $50.70 million, climbing to approximately $184.92 million as of September 30.

Why the Numbers Don’t Add Up—And Why They Do

The explanation lies in Mirum’s extraordinary stock performance. Shares have surged 89% over the past year, far outpacing the S&P 500’s more modest 15% gain. That price appreciation proved powerful enough to more than offset the reduction in share count.

After the portfolio adjustment, Mirum became Eventide’s second-largest equity holding, representing 3.04% of its assets under management (AUM), up from roughly 2.4% in the prior quarter. The position now ranks behind only Guardant Health (GH: $207.51 million) in the fund’s top holdings list:

• Guardant Health: $207.51 million (3.4% of AUM)
• Mirum Pharmaceuticals: $184.92 million (3.0% of AUM)
• Xometry: $169.87 million (2.8% of AUM)
• Thermostat Companies: $168.00 million (2.8% of AUM)
• Insmed: $144.74 million (2.4% of AUM)

Understanding Mirum’s Business Model

Mirum operates as a specialized biopharmaceutical company focused on rare and orphan liver diseases—a niche that commands premium valuations and patient loyalty. The company’s lead therapy, LIVMARLI, represents an investigational oral treatment for rare cholestatic liver conditions affecting patient populations like those with Alagille syndrome and progressive familial intrahepatic cholestasis.

As of the latest trading data, Mirum carried a market capitalization of $4.10 billion against trailing twelve-month revenue of $471.79 million. The company posted a net loss of $41.42 million during that same period, typical for biotech firms still scaling manufacturing and distribution.

Recent Performance Shows Commercial Momentum

Third-quarter results provided concrete evidence of business traction. Mirum reported Q3 revenue of $133 million, representing 47% year-over-year growth. Management raised full-year guidance to a range of $500 million to $510 million, driven predominantly by accelerating LIVMARLI sales and expanding bile acid medicine revenue streams.

Cash and investment reserves reached $378 million by quarter-end, providing substantial runway for pipeline development without immediate shareholder dilution pressures.

What the Share Trimming Actually Signals

Rather than indicating loss of conviction, Eventide’s decision to reduce share count while allowing the position to grow into a top-two holding suggests a different interpretation: portfolio rebalancing in the context of exceptional price appreciation.

The asset manager’s willingness to maintain an oversized position relative to other healthcare holdings signals confidence that Mirum represents genuine commercial transition rather than speculative momentum. This isn’t a trade designed to capture short-term volatility swings. Instead, it reflects an underwriting conviction that the business is transitioning from research promise into demonstrable commercial scale.

Within the broader context of Eventide’s portfolio construction, Mirum sits among other growth-oriented healthcare investments, reinforcing a manager philosophy that prioritizes companies with established commercial traction over binary clinical-outcome bets.

The Bottom Line

For equity holders monitoring institutional behavior, the newsworthy element here transcends the share sale itself. The more revealing signal is a major asset holder’s acceptance of an increasingly concentrated position during a period of rapid appreciation—a deliberate choice that speaks to fundamental confidence in Mirum’s trajectory from experimental stage toward sustainable, scalable revenue generation.

At $79.26 per share, Mirum offers one of the biotechnology sector’s more compelling case studies in how disciplined position management can coexist with bullish long-term conviction.