Before the two STAP cell papers were published in Nature in January of 2014, much of the same data was reportedly submitted as single papers to other high-profile journals including Science.
In these cases, the proto-STAP papers as we might call them were rejected.
Until now we largely could only speculate.
However, the reviews of the 2012 proto-STAP manuscript at Science can now be read at Retraction Watch.
As a result of reading the Science reviews, today we know what the reviewers at Science thought in 2012 of this proto-STAP paper and this sheds much light on what went so terribly wrong with STAP overall. There were many big red flags. Keep in mind that the Nature reviewers would not know about the Science reviews unless by chance one or more of the reviewers for Nature had also participated in the review process at Science.
Read the full, original story: Full reviews of rejected STAP paper point to early signs of big trouble