Upstream Bio, Inc.

SecurityUPB / Upstream Bio, Inc.
InstitutionMorgan Stanley
Latest Disclosed Ownership552,609 shares
Latest Disclosed Value $ 4,973,481
Morgan Stanley reports 352.71% increase in ownership of UPB / Upstream Bio, Inc.

On May 15, 2026 - Morgan Stanley filed a 13F-HR form disclosing ownership of 552,609 shares of Upstream Bio, Inc. (US:UPB) valued at $4,973,481 USD as of March 31, 2026. The entity filed a previous 13F-HR/A on May 27, 2026 disclosing 122,068 shares of Upstream Bio, Inc.. This represents a change in shares of 352.71% during the quarter. The current value of the position is $4,105,885 USD.

Institutional Ownership: 13F and NPORT Filings

The Security and Class in the table below are shown exactly as filed by the investor. We do our best to track continuity of investments through acquisitions, and this will be reflected in the table as changes in names. In addition to descriptive data, performance of the investment is shown over time. To calculate quarterly performance, we first calculate cost basis of the shares purchased during the quarter, then use that to calculate gross profit. Quarterly return is Gross Profit / Starting Portfolio value.

Note that cost basis is calculated and stored in thousands, so small quarterly changes in shares frequently result in a cost basis of zero.

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Disclosed Equity Positions (from 13F/NPORT Filings)
File
Date		 Effective
Date		Form Security Class ID Avg Price
Paid (Est)		 Reported
Shares		 ΔShares ΔShares
(%)		 Reported
Value
(x1000)		 ΔValue
(%)		 Alloc
(%)		 ΔAlloc ΔAlloc (%) Cost Basis
(x1000)		 Unrealized
Gain/Loss
(x1000)
2026-05-15 2026-03-31 13F UPSTREAM BIO COM 91678A107 552,609 430,541 352.71 4,973 50.06 0.0003
2026-05-27 2025-12-31 13F/A-1 UPSTREAM BIO COM 91678A107 122,068 34,533 39.45 3,314 101.34 0.0002
2026-02-13 2025-12-31 13F UPSTREAM BIO COM 91678A107 122,068 34,533 3,314 0.0002
2026-05-27 2025-09-30 13F/A-1 UPSTREAM BIO COM 91678A107 87,535 -11,773 -11.86 1,647 51.01 0.0001
2025-11-14 2025-09-30 13F UPSTREAM BIO COM 91678A107 87,535 -11,773 1,647 0.0001
2025-08-15 2025-06-30 13F UPSTREAM BIO COM 91678A107 99,308 47,917 93.24 1,090 247.13 0.0001
2025-05-15 2025-03-31 13F UPSTREAM BIO COM 91678A107 51,391 8,228 19.06 315 -55.71 0.0000
2025-05-15 2024-12-31 13F/A-1 UPSTREAM BIO COM 91678A107 43,163 43,163 710 0.0000
2025-02-14 2024-12-31 13F UPSTREAM BIO COM 91678A107 43,163 43,163 710 0.0000
Legend
Shares
The total number of shares held by the institution at the end of the reporting period (the effective date). This is provided in the filing.
Value
The total value of the shares as of the effective date. This is provided in the filing.
Avg. Share Price
The weighted average share price of the shares held by the institution. We use FIFO accounting to determine this price
Class
This is the security class as indicated by the filer. There are a variety of values for this field. Common values include "EC" = equity common, "EP" - equity preferred
Allocation (%)
The percent of the institution's portfolio that this position represents. This is provided in NPORT filings. For 13F filings, we calculate it.
Change in Allocation
The change in allocation from the prior reporting period. This is calculated as current allocation - prior allocation.
Change in Allocation (%)
The percent change in allocation from the prior reporting period. This is calculated as (current allocation - prior allocation) / prior allocation.
Cost Basis
The total cost of the shares held. This is calculated as reported shares * avg. share price
Unrealized Gain/Loss
This is the market value of the position as of the effective date minus the cost basis.