The estimated contribution of northern Han to southern Han is substantial in both paternal and maternal lineages and a geographic cline exists for mtDNA. As a result, the northern Han are the primary contributors to the gene pool of the southern Han. However, it is noteworthy that the expansion process was dominated by males, as is shown by a greater contribution to the Y-chromosome than the mtDNA from northern Han to southern Han. These genetic observations are in line with historical records of continuous and large migratory waves of northern China inhabitants escaping warfare and famine, to southern China. Aside from these large migratory waves, other smaller southward migrations occurred during almost all periods in the past two millennia. A study by the Chinese Academy of Sciences into the gene frequency data of Han subpopulations and ethnic minorities in China, showed that Han subpopulations in different regions are also genetically quite close to the local ethnic minorities, meaning that in many cases, blood of ethnic minorities had mixed into Han, while at the same time, the blood of Han had also mixed into the local ethnic minorities. A study on Armenian admixture in varied populations found 3.9% Armenian-like DNA in some northern Chinese Han.