A scientist has been making her case to be the first in the UK to be allowed to genetically modify human embryos.
Dr Kathy Niakan said the experiments would provide a deeper understanding of the earliest moments of human life and could reduce miscarriages.
The regulator, the Human Fertilisation and Embryology Authority (HFEA), will consider her application on Thursday.
If Dr Niakan is given approval then the first such embryos could be created by the summer.
Every person has gone through a remarkable transformation from a single fertilised egg into a fully fledged human being made of trillions of precisely organised cells.
Exactly how this takes place is a mystery.
Dr Niakan, who has spent a decade researching human development, is trying to understand the first seven days.
During this time we go from a fertilised egg to a structure called a blastocyst, containing 200-300 cells.
But even at this early blastocyst stage, some cells have been organised to perform specific roles - some go on to form the placenta, others the yolk sac and others ultimately us.
During this period, parts of our DNA are highly active.
It is likely these genes are guiding our early development but it is unclear exactly what they are doing or what goes wrong in miscarriage.
Dr Niakan, from the Francis Crick Institute, said: "We would really like to understand the genes needed for a human embryo to develop successfully into a healthy baby.
"The reason why it is so important is because miscarriages and infertility are extremely common, but they're not very well understood."
Of 100 fertilised eggs, fewer than 50 reach the blastocyst stage, 25 implant into the womb and only 13 develop beyond three months.
She says that understanding what is supposed to happen and what can go wrong could improve IVF.
"We believe that this research could really lead to improvements in infertility treatment and ultimately provide us with a deeper understanding of the earliest stages of human life."
Gene editing
However, she says the only way to do this is to edit human embryos.
Many of the genes which become active in the week after fertilisation are unique to humans, so they cannot be studied in animal experiments.
"The only way we can understand human biology at this early stage is by further studying human embryos directly," Dr Niakan said.
Her intention is to use one of the most exciting recent scientific breakthroughs - Crispr gene editing - to turn off genes at the single-cell stage and see what happens.
She said that if our DNA were an entire encyclopedia, then Crispr had the precision to alter a single letter on a single page.
She aims to start with the gene Oct4 which appears to have a crucial role.
It is expressed only in a handful of blastocyst cells (pictured above in green) which go on to form all the tissues of the human body.
She will stop it functioning in 20-30 donated embryos and if the experiments are successful her research group will move on to other genes.
The knowledge gained from such studies could help pick which embryos had the best chance of resulting in a successful pregnancy in IVF.
Such experiments are legal in the UK as long as the modified embryos are not implanted into people.
But scientists need a licence from the HFEA before they can perform such studies.
The regulator will consider Dr Niakan's request on Thursday.
The field is attracting controversy with some saying that altering the DNA of an embryo is a step too far.
Dr David King, the director of the campaign group Human Genetics Alert, said: "This is the first step on a path that scientists have carefully mapped out towards the legalisation of GM babies."
Dr Niakan said this was not the aim of her research and that: "In the UK there are very appropriately tight regulations in this area that would make it completely illegal to move it in that direction [of genetically modified babies]."
Wolf Reik, a genetics professor at the Babraham Institute in Cambridge, said such experiments needed to be carefully monitored and regulated to prevent misuse, but they were an "exciting prospect".
He added: "The long term impacts on understanding human development and hence on better regenerative medicine approaches will be considerable.
"Not everything about human development can be understood from studying model organisms such as mice or even non-human primates."
James Gallagher - BBC News
