MUU Short Interest Hits 355% of Float as Borrow Market Locks Up

Short sellers have piled into MUU at a pace rarely seen in leveraged ETF markets. Short interest jumped from roughly 1 million shares on July 13 to 14.3 million shares on July 15 — a 4,895% surge in one week, pushing estimated short interest to 355% of float.

That number is not a typo. For a 2x leveraged single-stock ETF tracking Micron Technology, short interest exceeding the entire float reflects the mechanics of leveraged product creation and redemption. But the scale of the move in a matter of days signals something unusual is happening in the borrow market.

Availability Has All But Vanished

The borrow market tells the clearest story. Availability — the ratio of shares still available to lend relative to shares already borrowed — sits at just 5.8%. That means roughly one share remains available for every seventeen already out on loan. The lending pool is effectively exhausted.

This tightness has been persistent. Availability dropped below 12% on July 10 and has stayed there. On July 7 it briefly touched 1.9%. The 52-week low is 0%, hit multiple times between June and mid-July.

Despite that tightness, cost to borrow has fallen sharply. CTB dropped to 1.96% on July 15, down 53% over the prior week. That divergence — maximum borrow stress with falling borrowing cost — suggests the demand for new shorts has cooled even as existing positions remain locked in.

The Leveraged ETF Dynamic

MUU tracks twice the daily return of Micron Technology. The ETF is down 28% over the past week and 45% over the past month. That sustained decline reflects Micron's weakness amplified through the 2x daily reset mechanism.

Short interest exceeding 100% of float is structurally possible in leveraged ETFs. Creation and redemption activity can result in short positions that exceed the technically outstanding share count. Still, the 1,318% single-day jump on July 15 — from ~1 million shares to 14.3 million — is an extreme single-session move.

The ORTEX short score stood at 46.6 as of July 15, down sharply from 65.1 on July 13. That drop reflects the changing composition of signals rather than a straightforward easing of pressure.

What the Options Market Shows

Options positioning is relatively calm by comparison. The put/call ratio sits at 0.82 on July 16, near but below the 52-week high of 0.97. The PCR z-score of 0.38 indicates modest bearish skew — elevated but not extreme.

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• Short interest (July 15): 355% of float, up 4,895% over one week
• Availability: 5.8% — near the tightest level in 52 weeks
• Cost to borrow: 1.96%, down 53% week-on-week
• Price: $27.51, down 28% in one week, down 45% in one month
• PCR: 0.82, z-score 0.38
• Short score: 46.6 (July 15), down from 65.1 two days prior