Scientists believe that in two years time they could "de-extinct" mammoths by creating a hybrid embryo.
Key traits of the huge mammal - such as long, shaggy hair, thick layers of fat, and cold-adapted blood - would be programmed into an Asian elephant, marking a turning point in bringing animals back from extinction.
However, the creation of a living creature remains many years away.
US scientists have ambitious plans to grow the hybrid embryo in an artificial womb, rather that use an elephant as a surrogate mother.
Since starting the project in 2015 the researchers have increased the number of "edits" where mammoth DNA has been spliced into the elephant genome from 15 to 45.
Creating what Professor George Church, who heads the Harvard University team, described as "an elephant with mammoth edits".
Prof Church continued: "We're working on ways to evaluate the impact of all these edits and basically trying to establish embryogenesis in the lab.
"The list of edits affects things that contribute to the success of elephants in cold environments.
"We already know about ones to do with small ears, sub-cutaneous fat, hair and blood, but there are others that seem to be positively selected."
The woolly mammoth roamed across Europe, Asia, Africa and North America during the last Ice Age and vanished some 4,500 years ago, probably due to a combination of climate change and hunting by humans.
Their closest living relative is the Asian, not the African, elephant.
The hybrid embryos would be created by "cutting and pasting" strands of DNA from mammals frozen thousands of years ago in Siberian ice.
The scientists intend to engineer elephant skin and take their reprogrammed nuclei - containing the mammoth edits -, which would then be placed into elephant egg cells whose own genetic material has been removed. These eggs would then be artificially stimulated to develop into embryos.
These embryos would be grown in an artificial womb since Asian elephants are endangered and "t would be unreasonable to put female reproduction at risk in an endangered species," explained Prof Church.
Prof Church helped develop the most widely used gene editing technique known as CRISPR/Cas9, which has transformed genetic engineering since it was first demonstrated in 2012.
Prof Church said the mammoth project had two goals - securing an alternative future for the endangered Asian elephant and helping to combat global warming.
Woolly mammoths could help prevent tundra permafrost (permanently frozen soil a foot or two below the surface) from melting and releasing huge amounts of greenhouse gas into the atmosphere.
Mammoths could "keep the tundra from thawing by punching through snow and allowing cold air to come in," said Prof Church.
"In the summer they knock down trees and help the grass grow."
A simulated ecosystem study has indicated that mammoths in Siberia could cause local temperatures to drop by up to 20C.