haplogroup Y [ = 21.6% according to Tanaka et al. 2004, or = 19.6% according to Adachi et al. 2009, who have cited Tajima et al. 2004], haplogroup D [ = 17.6%, particularly D4 (xD1)], haplogroup M7a ( = 15.7%), and haplogroup G1 ( = 15.7%). Other mtDNA haplogroups detected in this sample include A (), M7b2 (), N9b (), B4f (), F1b (), and M9a (). Most of the remaining individuals in this sample have been classified definitively only as belonging to macro-haplogroup M. According to Sato et al. (2009), who have studied the mtDNA of the same sample of modern Ainus (=51), the major haplogroups of the Ainu are N9 [ = 27.5%, including Y and N9 (xY)], D [ = 23.5%, including D (xD5) and D5], M7 ( = 19.6%), and G ( = 19.6%, including G1 and G2); the minor haplogroups are A (), B (), F (), and M (xM7, M8, CZ, D, G) (). Studies published in 2004 and 2007 show the combined frequency of M7a and N9b were observed in Jōmons and which are believed by some to be Jōmon maternal contribution at 28% in Okinawans [ M7a1, M7a (xM7a1), N9b], 17.6% in Ainus [ M7a (xM7a1), N9b], and from 10% [ M7a (xM7a1), M7a1, N9b] to 17% [ M7a1, M7a (xM7a1)] in mainstream Japanese. In addition, haplogroups D4, D5, M7b, M9a, M10, G, A, B, and F have been found in Jōmon people as well. These mtDNA haplogroups were found in various Jōmon samples and in some modern Japanese people. A study by Kanazawa-Kiriyama in 2013 about mitochondrial haplogroups, found that the Ainu people (including samples from Hokkaido and Tōhoku) have a high frequency of N9b, which is also found among Udege people of eastern Siberia, and more common among Europeans than Eastern Asians, but absent from the geographically close Kantō Jōmon period samples, which have a higher frequency of M7a7, which is commonly found among East and Southeast Asians. According to the authors, these results add to the internal-diversity observed among the Jōmon period population and that a significant percentage of the Jōmon period people had ancestry from a Northeast Asian source population, suggested to be the source of the proto-Ainu language and culture, which is not detected in samples from Kantō. A study by Adachi et al. 2018 concluded that: "Our results suggest that the Ainu were formed from the