% of shares outstanding not shown: no SEC cover-page share count falls within 120 days of the 2026-Q1 quarter end, so any percentage would rest on a count that cannot speak for this quarter. Holder share counts are shown as filed.
Crowding percentile withheld — no market cap for ARIMX in the served universe — the cohort is size-matched, and an unmatched percentile would compare a micro-cap register to a mega-cap one.
| # | Holder | Shares | Value | % of shares out | Δ QoQ |
|---|---|---|---|---|---|
| 1 | Hemenway Trust Co LLC | 1,830,9751.8M | $18M | — | — |
| 2 | Broderick Brian C | 673,406673,406 | $7M | — | — |
| 3 | SOMERVILLE KURT F | 260,567260,567 | $3M | — | — |
| 4 | Dempze Nancy E | 252,648252,648 | $2M | — | ADD 5% |
| 5 | Elefante Mark B | 68,29368,293 | $667,912 | — | ADD 64% |
| 6 | Delaney Dennis R | 20,24520,245 | $197,994 | — | NEW |
Rows rank by the value each filer reported — 13F filers price their own positions, so a holder with fewer shares can rank just above one with more.
| Holder | 2024-Q4 | 2025-Q1 | 2025-Q2 | 2025-Q3 | 2025-Q4 | 2026-Q1 | Latest value |
|---|---|---|---|---|---|---|---|
| Hemenway Trust Co LLC | 1.7M | 1.9M | 1.9M | 1.8M | 1.8M | 1.8M | $18M |
| Broderick Brian C | 673K | 673K | 673K | 673K | 673K | 673K | $7M |
| SOMERVILLE KURT F | 207K | 274K | 276K | 266K | 266K | 261K | $3M |
| Dempze Nancy E | 228K | 231K | 233K | 237K | 242K | 253K | $2M |
| Elefante Mark B | 29K | 30K | 30K | 42K | 42K | 68K | $667,912 |
| Delaney Dennis R | — | — | — | — | — | 20K | $197,994 |
Shares reported each quarter by the largest current holders. Green = added vs the prior filing, red = trimmed, grey = roughly held; a dash is a quarter with no 13F on file (a gap is skipped, not read as an exit). 13F is filed up to 45 days after quarter-end. Up to 40 quarters (10 years) available — use the selector above.
Holder count only — too few computable %-of-shares-outstanding quarters for a % pane. Line breaks are quarters with no 13F record, never interpolated.
| Quarter | Holders | Shares held | % shares out | Value held |
|---|---|---|---|---|
| 2026-Q1 | 6 | 3,106,134 | — | $30M |
| 2025-Q4 | 5 | 3,039,771 | — | $30M |
| 2025-Q3 | 5 | 3,036,093 | — | $31M |
| 2025-Q2 | 5 | 3,154,664 | — | $32M |
| 2025-Q1 |
Short interest has its own tab now — open SI →
Source: SEC Form 13F — every institution manager (≥ $100M in 13F securities) reporting a long US-listed position, across ~10,250 filers over 10 years of history. 13F is long-only, US-listed, filed up to 45 days after quarter-end.
| Biggest buyers | Δ shares | Δ value | |
|---|---|---|---|
| Elefante Mark B | +26,608 | $260,226 | ADD |
| Delaney Dennis R | +20,245 | $197,996 | OPEN |
| Hemenway Trust Co LLC | +13,886 | $135,805 | — |
| Dempze Nancy E | +10,902 | $106,622 | ADD |
| Biggest sellers | Δ shares | Δ value | |
|---|---|---|---|
| SOMERVILLE KURT F | −5,278 | $51,619 | — |
The 2025-Q4 register is as-filed in ITS OWN share basis; every count below was rebased into the 2026-Q1 basis (no split in between, factor 1). Dollar figures value the SHARE delta at one price — the 2026-03-31 close — rather than differencing two filings' own reported values, which would mix the quarter's price move into the trade figure.
13Fs disclose LONG US-listed positions only — shorts, most derivatives, bonds and non-US listings are invisible here, and a filer's quarter-over-quarter change is not necessarily a trade (index rebalances, in-kind transfers and entity successions all read as flow).
| 5 |
| 3,075,450 |
| — |
| $30M |
| 2024-Q4 | 5 | 2,800,463 | — | $28M |
| 2024-Q3 | 4 | 2,499,773 | — | $26M |
| 2024-Q2 | 4 | 2,281,865 | — | $22M |
| 2024-Q1 | 4 | 2,200,881 | — | $23M |
| 2023-Q4 | 4 | 1,679,308 | — | $16M |
| 2023-Q3 | 4 | 1,156,507 | — | $11M |
| 2023-Q2 | 3 | 1,095,751 | — | $10M |
| 2023-Q1 | 4 | 921,881 | — | $8M |
| 2022-Q4 | 4 | 859,070 | — | $8M |
