Protein-sequencing company Quantum-Si has committed to 54,374 square feet at Pacific Center in Sorrento Mesa under a 10-year lease that carries 20 months of free rent and a $17.1 million tenant improvement allowance, becoming the first tenant at a 525,000-square-foot campus that Sterling Bay and Harrison Street refinanced with $290 million in May.
The most expensive empty building in Sorrento Mesa finally has a tenant.
Quantum-Si has leased approximately 54,374 square feet of office, laboratory and manufacturing space at 9955 Pacific Heights Boulevard, the Sterling Bay and Harrison Street life sciences campus known as Pacific Center, according to a Form 8-K the company filed with the Securities and Exchange Commission. The lease was signed June 18, 2026, with Sterling City Science South Development LLC, a Sterling Bay affiliate.
The deal was the largest lease recorded in San Diego’s life sciences market during the second quarter, according to Savills’ Q2 2026 Market in Minutes report, which lists Quantum-Si as the region’s top tenant transaction.
Terms tell the story of a market in which tenants set the price. The lease runs 120 months with one five-year extension option, commencing on or about September 1, 2027 and no later than November 1, 2027. Initial base rent is roughly $315,369 per month, or approximately $5.80 per square foot, escalating 3 percent annually. Quantum-Si receives 20 months of rent abatement and a tenant improvement allowance of up to $17.1 million, according to the filing.
That allowance works out to roughly $314 per square foot. Landlords do not write checks like that in strong markets.
They write them in markets like this one. San Diego’s overall life sciences vacancy stood at 28.1 percent at the end of the second quarter, essentially unchanged from 28.2 percent a year earlier, while available space climbed to 9.4 million square feet from 8.9 million square feet, according to Savills. Sorrento Mesa carries the region’s heaviest overhang; Cushman & Wakefield pegged direct availability in the submarket at 42.9 percent in its Q1 2026 report.
Quantum-Si develops single-molecule protein sequencing instruments. The company, which went public through a special purpose acquisition merger in 2021, is headquartered in Branford, Connecticut, and the San Diego space is not a relocation from the East Coast. It replaces existing San Diego space whose lease expires December 31, 2027, and it will house work on Proteus, the company’s next-generation sequencing platform targeted for commercial launch at the end of 2026.
The commitment lands as Quantum-Si operates under the discipline of a shrinking headcount. The company disclosed a workforce reduction of approximately 16 percent and reported roughly $230 million in cash as of the third quarter of 2025, with runway guided into the second quarter of 2028, according to its SEC filings. Signing a 10-year lease that begins in late 2027 is a bet that Proteus reaches commercial scale before the rent does.
For Sterling Bay and Harrison Street, the lease closes a loop three years in the making.
The partnership acquired a five-building Sorrento Mesa life sciences portfolio plus a 13-acre development site in December 2021, part of a commitment the firms valued at roughly $1.6 billion. Pacific Center rose on the development parcel: two six-story lab and office buildings totaling about 525,000 square feet, LEED Gold certified, delivered in 2025 as the first phase of a planned 1.1-million-square-foot campus.
Then it sat empty. Bank OZK’s $265 million construction loan, originated in 2023, traded to Strategic Value Partners earlier this year. In May 2026, Sterling Bay and Harrison Street secured $290 million in refinancing from Peregren Capital Group, arranged by Newmark’s Jonathan Firestone, Blake Thompson and Jack Condon, according to Newmark. The package included $162.5 million in future funding earmarked for lease-up.
Lenders reserve future funding for buildings they expect to fill. Six weeks later, Quantum-Si signed.
The sequencing is not coincidental. Refinancing proceeds fund the tenant improvement allowances that win leases in an oversupplied market, and an anchor lease is what persuades a lender to commit those proceeds. Sterling Bay executed both halves of that trade inside a single quarter.
What Pacific Center does not yet have is a second tenant. Quantum-Si takes the entire sixth floor of one building, roughly 10 percent of the delivered campus. The remaining 470,000 square feet compete against 9.4 million square feet of available space in a region where venture funding into life sciences companies fell to $500 million in the second quarter from $600 million a year earlier, according to PitchBook data cited by Savills.
Sterling Bay has 20 months of Quantum-Si’s free rent to find the next signature. It also has something no competing landlord in Sorrento Mesa can claim: a credit tenant on the rent roll, a recapitalized balance sheet, and $162.5 million of committed capital to spend chasing the rest.
