We’ve all heard that non-African humans have 1-3% Neandertal DNA, but some folks at UW Seattle have found that some 20% of the Neandertal genome can be found scattered around in moderns. They used a database of ~700 Europeans and East Asians and some statistical methods that I haven’t yet checked up on to distinguish introgressed vs nonintrogressed sequences, so without actually matching modern genomes to Neandertal genomes. They did check to make sure their methods flagged known Neandertal sequences.
I’m probably not going to dig into the math enough to evaluate the work. They do say there are likely many false-positives, so we’ll see some refinement down the road.
What’s most interesting to me is that they see very few Neandertal sequences in some chunks of the genome, like those that are thought to help with speech and language. They see an increased frequency in genes related to skin pigmentation. They also believe that modern humans received multiple (at least two) pulses of Neandertal genes, both before and after European and Asian populations split (with Asians getting the 2nd.)
Actual paper at 10.1126/science.1245938
Non-walled summary at http://www.washington.edu/news/2014/01/29/neanderthal-lineages-excavated-from-modern-human-genomes/