Baker Bros. 13F: Q1 2026 Biotech Holdings

What did Baker Bros.' latest 13F show?

The latest effective Baker Bros. Advisors filing in Arkolith's 13F corpus is the Q1 2026 Form 13F-HR for the quarter ended March 31, 2026. The filing was accepted by the SEC on May 15, 2026 at 20:12:32 UTC under accession number 0001104659-26-062547. The primary record is the SEC's SEC filing detail page.

According to Arkolith's production parse of that accession, Baker Bros. reported $17.416 billion across 84 positive non-option lines. Unlike several fund filings, this one had no positive option positions in the effective book, so the filing aggregate equals the non-option reported value used in the tables below.

Source: Baker Bros. Advisors Form 13F-HR for Q1 2026, SEC accession 0001104659-26-062547, parsed from EDGAR and queried from Arkolith production on August 5, 2026.

How concentrated was the Baker Bros. book?

The top-three positive-long lines, Incyte, BeOne Medicines, and Insmed, represented about 38.6% of the reported non-option book. The top ten represented about 73.5%. That is a concentrated specialist filing: a handful of biotech and pharma lines carry most of the public 13F value.

The concentration matters because it changes how to read the filing. A 10-stock move can dominate the signal, while many smaller development-stage positions function more like a watchlist of clinical, regulatory, or financing exposure. Baker Bros.' 13F is not a broad-market factor sleeve. It is a source-backed snapshot of a biotech-heavy long book.

Which Baker Bros. positions were new?

Mirum Pharmaceuticals was the largest new positive-long line in the Q1 comparison. Baker Bros. reported 445,975 shares worth about $41.2 million. Other new disclosed lines included Evommune, Aktis Oncology, Generate Biomedicines, COMPASS Pathways, Eikon Therapeutics, and Verastem.

A new 13F line proves the position was reportable at quarter end. It does not prove the trade date, current holding, position intent, or whether the manager held related exposure outside the 13F reporting universe.

Which holdings increased or decreased most?

The largest share-count addition among continuing positions was AbCellera Biologics. Baker Bros. reported 30,275,938 shares, up 2,750,298 shares, or 10.0%, from the prior comparable filing. GRAIL almost doubled by share count, rising 93.2% to 4,028,614 shares. Insmed added 828,365 shares, while Crinetics and Praxis also rose meaningfully by share count.

The largest share-count reduction was Immunocore Holdings, down 1,588,259 shares, or 68.4%, to a $22.1 million line. Immatics fell 9.7%, Roivant fell 12.4%, Tectonic Therapeutic fell 32.7%, BioNTech fell 67.5%, and Arrowhead fell 8.4%.

What disappeared from the latest filing?

Several prior positive-long lines were absent from the Q1 2026 positive-long book. The largest by prior reported value was Ultragenyx Pharmaceutical, which had been a $63.8 million line in the prior comparable filing. Other absent lines included Terns Pharmaceuticals, BioCryst Pharmaceuticals, Ventyx Biosciences, Nuvalent, 4D Molecular Therapeutics, Vir Biotechnology, KALA BIO, and Acrivon Therapeutics.

An absence should be read carefully. It supports an exited-reportable-long read for the latest parsed 13F book. It does not rule out exposure through shorts, swaps, cash-settled instruments, private holdings, or securities outside Form 13F's reportable list.

How should readers interpret a Baker Bros. 13F?

A 13F is useful because it creates a public, source-backed institutional holdings snapshot. It is not live portfolio surveillance. The SEC's SEC Form 13F FAQ describes the form as a quarterly report for institutional investment managers that meet the Section 13(f) threshold. The Form 13F instructions set the filing deadline at 45 days after quarter end.

That clock is the boundary. Baker Bros.' Q1 filing was accepted on May 15, 2026, and it describes reportable holdings as of March 31, 2026. It does not prove what the manager owned in June, July, or today. It also omits shorts, cash, many derivatives, private positions, some foreign securities, and post-quarter activity.

The strong read is narrower: the public Q1 source record shows a large biotech book concentrated in Incyte, BeOne Medicines, Insmed, Madrigal, ACADIA, Revolution Medicines, Celcuity, Kodiak, Kymera, and Summit, with Mirum as the largest new disclosed line and Ultragenyx absent from the latest positive-long book.

Where can I verify Baker Bros.' 13F?

Start with the SEC accession page, then inspect the rendered fund page. The SEC page proves the filing exists, its form type, acceptance time, and source documents. The Baker Bros. fund page keeps the latest holdings, historical filings, position changes, and source links in one place.

For a programmatic workflow, resolve the fund by CIK 1263508, pull the latest holdings, and keep the accession number beside each position. That source-carrying pattern is the practical reason to use a structured 13F holdings API or the MCP quickstart instead of asking a model to remember a biotech portfolio from training data.

If the next step is an agent workflow rather than a manual read, connect the data layer, then ask for Baker Bros.' latest holdings with source accessions and quarter-over-quarter changes returned beside each row.

What should an analyst watch next?

The useful follow-up is whether the next public filing confirms or reverses the Q1 signals. For Baker Bros., the next 13F can answer several specific questions:

Those watch points are better than a generic copy-trade prompt because they state the uncertainty before the next filing arrives. The next filing can answer whether the lines changed. It cannot answer why unless Baker Bros. says so in a separate primary source.

Frequently asked questions about Baker Bros.' 13F

What is Baker Bros.' 13F filer CIK?

The Baker Bros. Advisors 13F filer in this article is CIK 1263508. The Q1 2026 accession used here is 0001104659-26-062547.

What was Baker Bros.' largest Q1 2026 13F holding?

Incyte was the largest reported line by value at about $2.89 billion, or about 16.6% of the Q1 2026 filing aggregate.

Did Baker Bros. buy Mirum in Q1 2026?

The effective 13F comparison shows Mirum Pharmaceuticals as a new reported stock line with 445,975 shares and about $41.2 million of reported value. A 13F proves the quarter-end disclosed line, not the exact trade date.

Did Baker Bros. sell Ultragenyx?

Ultragenyx appeared in the prior positive-long book and was absent from the latest parsed positive-long book. That supports an exited-reportable-long read, but it does not rule out exposure outside reportable 13F long securities.

Is this the whole Baker Bros. portfolio?

No. Form 13F covers reportable U.S. long securities and certain option positions. It omits shorts, cash, many derivatives, private holdings, non-reportable securities, and current changes after quarter end.

This article reports public regulatory filings and data workflows. It is not investment advice, legal advice, tax advice, or accounting advice.