Yamnaya and Corded Ware had some of this Dnieper/Ukraine ancestry but less than most Sredny Stog samples do.

In the competition for modern pops closest to Proto-Indo Euroepans, it is close between Europeans as a whole and the tiny inbred pops in the northern Caucasus.

Some Steppe groups are slightly closer to northern Caucasus, some slightly closer to Europeans

When talking about large regions, Europe is the region closest to PIE.

Northern Caucasus is a weird tiny region that doesn't really count

U5 is an mtDNA haplogroup that over 50% of Mesolithic Europeans belonged to. Today about 10% do. U5's frequencies are lower today because the majority of modern Europeans have Anatolian farmer mtDNA.

Even Finnish with only about 20-25% Anatolian farmer ancestry have over 50% Anatolian-farmer derived mtDNA. Literally all modern European pops have mostly Anatolian mtDNA.

Maternally modern Europeans are mostly Anatolian farmer and paternally mostly Mesolithic European

U5 was frequent long before the Mesolithic. It has been frequent going back at least 30,000 years.

Me and STJ have U5.

In my profile pic I was trying to imitate this depiction of Odin. The one eyed god.

Infographic about mtDNA haplogroups in Mesolithic Europe

Most but not all Mesolithic mtDNA variation can be explained by the WHG and EHG categories. WHG population in the Balkans had a high frequency of mHG U8 while most other WHG populations had a low frequency or lacked it. WHG populations in Italy had a high frequency of U10 but this haplogroup is not found in most other WHG populations.

U5 for sure comes from "WHG". U4 for sure comes from "ANE"
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Interestingly, Mesolithic Europe, in particular the Balkans, shared haplogroup U8 with Anatolian farmers who would later migrate into Europe. They even shared the same subclade U8b2a (K1) which originated in about 20,000 BP

U8 was the most common haplogroup in Anatolian farmers

U8 is probably connected to the so called WHG/Euro related ancestry present in Anatolian farmers