Double Duty: Complete Pathologic Response of Two Colonic Primaries with Mosaicism of a Novel MLH1 Mutation to Neoadjuvant Pembrolizumab.

We present a fascinating case of a 57-year-old male with a novel mutation in MLH1 ( MLH1:c.1288G > T, p.(Glu430*) ), who presented with two synchronous colonic tumours, initially deemed unresectable, and experienced a complete pathological response on neoadjuvant pembrolizumab. Extensive genetic testing revealed post-zygotic mosaicism from the novel mutation.