Yu Hong (d. 592 [C.E.]) was a high-ranking member of a community of Sogdians who had settled on the northern border of China at the beginning of the fourth century. While barely in his teens, Yu Hong began his career in the service of the most powerful nomadic tribe at the time, known as the Ruru, and was posted as an emissary to several countries, including Iran.
Now, the genetics:
...we discuss our analysis of the mitochondrial DNA of human remains excavated from the Yu Hong tomb in Taiyuan, China, dated 1400 years ago. The burial style of this tomb is characteristic of Central Asia at that time. Our analysis shows that Yu Hong belonged to the haplogroup U5, one of the oldest western Eurasian-specific haplogroups, while his wife can be classified as haplogroup G, the type prevalent in East Asia. Our findings show that this man with European lineage arrived in Taiyuan approximately 1400 years ago, and most probably married a local woman. Haplogroup U5 was the first west Eurasian-specific lineage to be found in the central part of ancient China
Note that this was the man's female lineage, his mtDNA. His Y might tell a different story. The importance of a U5 finding is that it is a very west Eurasian lineage, so this implies some major continent-hopping. We know from the historical data, textual and archaeological, that "Europoid" individuals were extant in western and central China in the past. But how common were they? In what regions and which social strata were they concentrated? Archaic DNA extraction can help us fill in the blanks, or add clarity to our fuzzy outlines. Of course you might ask, but didn't we know this already? Yes, we can be fairly certain without genes that Europoid individuals were resident in China. But, "fairly certain" is different from "totally certain," and the nature of archaeology and text often results in inaccuracies and sample bias. Genetics is not immune from this bias, but, if they are different biases then one presumes that a sort of complementation might occur which gives us a better picture of the past. Finally, though we may have a consensus there may always be a minority view which contends that the archaeology (e.g., wall paintings, statuary) are signatures of a cultural diffusion of idealized motifs, while the textual data is skewed by exaggeration on the part of the Chinese. This is not a critique without grounding, if you looked at public art in Bangladesh you would assume that the modal skin color is olive-brown, instead of medium to dark brown, because of social preference for light skin. Similarly, if you read pre-modern Chinese texts you might expect all Westerners to literally have red hair and green eyes. Genes are another way to quantify and characterize the distribution of traits in the past. Also, please note similarities to the Etruscan genetics story. The two populations in question are very distinct phylogenetically, brought together by rapid long distance travel (e.g., horse & ship respectively). And, we have independent lines of evidence corroborating and indicating the hypotheses that we seek to test. My own sense is that historical population genetics will be more fruitful acting as the bloodhound, seeking the scent of targets sighted by other disciplines, as opposed to attempting to go it alone and extracting history purely from gene genealogies. At least when it comes to neutral genetic markers of ancestry as opposed to selectively driven ones which indicate adaptation. Related: My post Romans in China suggests that Europoid characters remain extant in portions of China proper.