The design earns its result by holding the molecule still. The same single molecules are probed round after round with fluorescently labeled antibodies [1], so what comes out is a per-molecule pattern rather than a population average. That is precisely what bottom-up mass spectrometry surrenders when it digests proteins into peptides: the evidence that two modifications sat on the same chain [10].
Twelve site-specific antibodies [2] read as present or absent define 4,096 possible patterns, which makes the 130 groups reported [8] about three percent of the accessible space [20]. The ratio matters in both directions. Tau's observed combinatorial diversity sits well below its theoretical ceiling, and that ceiling is set by the panel rather than by tau: modifications outside those twelve epitopes fall outside the census entirely. Top-down mass spectrometry, which keeps proteins intact, retains the one thing an antibody panel cannot offer, the ability to find modifications nobody thought to probe for [14]. There is also still no comprehensive reference database of human proteoforms to check assignments against [13].
The sensitivity figure is what makes frequency arguments possible. A proteoform at 0.1 percent abundance [3] turns up roughly a thousand times in a million-molecule readout [21], enough to compare how often combinations occur rather than merely note that they exist. A median coefficient of variation below 5.5 percent [4] is precision across replicates, and it says nothing about whether each antibody bound the site it is named for. That dependency has history: the commentary accompanying the paper, by Albert Heck, lists the 2011 assessment of histone-modification antibody quality among its references [16][17].
The ordering result is the interesting science, and it carries a condition. A non-random distribution of phosphorylation events across proteoform groups [9] is what ordered, site-specific modification would produce, and the paper places the order and timing of modification among the field's open questions [15]. The census captures a single moment, leaving the sequence in time unresolved. Combination frequencies also depend on how long each proteoform persists before degradation and on which cells contributed molecules to the sample. A snapshot in which AB and ABC are common while AC is rare fits ordered writing, and it fits faster clearance of molecules carrying C. A time course would separate those.
Read as a neurodegeneration assay rather than a methods advance, the picture is narrower. Human Alzheimer's material was analysed alongside organoids and mouse brain [7], and the authors report the assay resolving closely related proteoform groups better than conventional techniques [6], which is the specific weakness of bulk phospho-tau measurement. The reported figures show analytical performance and a group count, not donor-level discrimination between cases and controls [22]. The same commentary cites recent mass spectrometry work on the tau proteoform landscape of human brain and on molecular features that distinguish tauopathy-associated dementias [19], which is the literature any disease-level claim from this assay will have to be measured against.
