When Regeneron, one of the world's largest biotech companies, opened a lawsuit against Kymab, a British biotech pioneer, back in 2013, it was the start of a David and Goliath legal battle, that came to be one of the most hotly contested patent disputes.

The case was filed by Regeneron for infringement of its patents covering mice that contain the so-called 'reverse chimaeric' immunoglobulin (i.e. antibody) gene loci, which comprise a human variable region and a mouse constant region.

At first instance, London's High Court held that, whilst Kymab's Kymouse did infringe Regeneron's patents, they were invalid seeing as none of the methods disclosed in the patents would have allowed the skilled person to make any of the mice within the claimed range without undue burden.  However, that decision was later to be overturned by the Court of Appeal, which held that: