What PC9 separates: a comment on Chobanov and Stamov (2025)

Chobanov and Stamov (2025) screen all twenty principal components of a curated ancient DNA set and report that PC9 discriminates early Slavic from Germanic ancestry where PC1-2 cannot, building on it an argument for three Slavic migration waves and for the Avar Khaganate as the setting of South Slavic ethnogenesis. This comment tests that claim against Supplementary Table S21, the PCA eigenvector file deposited with the article, which gives PC1-PC20 for 3,607 individuals. Three findings. The European maximum of PC9 reported in the paper is exceeded by seven European individuals in the deposited file, six of them Avar-period burials from the Danube-Tisza interfluve and one a Bronze Age Yamnaya individual from Ukraine; four of these appear in the authors' own supplementary table, but in its Eurasian rather than its European panel. A two-sided Mann-Whitney test against the 94 explicitly labelled Germanic Migration Period individuals in the file ranks PC9 ninth of twenty components, and widening the Slavic comparison set using populations the paper itself treats as Slavic-associated removes the effect entirely. The published screening function scores components solely by the extremity of the Slavic reference, with no contrast population entering the calculation, so it cannot test the discrimination it is used to establish. The paper describes a quality-controlled dataset of 1,800 individuals; the deposited eigenvector file contains 3,607.