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The Direxion Daily MU Bull 2X ETF (MUU) has become the fourth single-stock ETF to cross $5 billion in assets under management.The fund's assets have grown more than tenfold since the start of the year, from $516 million to $5.4 billion, on $1.3 billion of inflows and a soaring share price for Micron, which is up 230% in 2026.MUU itself is up 712%, just shy of the Breakwave Tanker Shipping ETF (BWET) and its 743% gain, the best-performing ETF of the year.Only four single-stock ETFs have ever reached $5 billion. The GraniteShares 2x Long NVDA Daily ETF (NVDL) was first, in 2024, followed later that year by the Direxion Daily TSLA Bull 2X ETF (TSLL). The YieldMax MSTR Option Income Strategy ETF (MSTY) joined in 2025, and now MUU.Of those four, only TSLL still sits above the mark, with $5.7 billion. NVDL is back around $4.5 billion, while MSTY has slid all the way to $1.3 billion.

Without consistent price gains or inflows, single-stock ETFs have trouble holding onto assets. The leveraged ones get worn down by volatility decay and the financing costs that come with leverage. Options-based funds like MSTY face a different problem of capped upside and near-full exposure to the downside, which bleeds the fund whenever the underlying stalls or falls.For now, MUU is on the right side of all of that, with strong price gains and inflows. Micron has been riding a severe bottleneck in high-bandwidth memory, a key component in AI data centers. On Tuesday alone the stock jumped more than 19%, lifting MUU by 38%.Micron crossed a $1 trillion market cap that day after UBS analysts raised their price target to $1,625, more than double where the stock was trading when the note went out.The average 12-month price target sits at $722, below the current $935 share price. But analysts have been behind the curve on both the stock's run and the company's earnings power.Everything is working in favor of the trade right now. I've covered how the Roundhill Memory ETF (DRAM) rode the same memory boom to $12 billion in assets in just two months. MUU is a very different and far riskier animal given the leverage, but it's ballooning in size nearly as fast.

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