Very little. He did some work on what would now be called mutational "recovery" of frame shift insertion/deletion variants in phages, for about a year on sabbatical. Which at the time would have been enough for him to have predicted several aspects about the genetic code, such as DNA triplets forming codons. Which was funnily enough anticipated by George Gamow around the same time looking at the number of naturally occurring amino acids and the number of nucleobases.