This was not the feared 226L mutation: The amino acid had changed to a histidine instead of leucine.

Notably, the increase in activity was specific to increasing concentrations of ATP or histidine; no changes were observed in response to other nucleotide triphosphates or amino acids.

The felines showed a particular preference for bowls containing histidine and inosine monophosphate—compounds found at particularly high levels in tuna.

The sequencing data revealed a variant that substituted an amino acid called histidine for arginine.

Me-His is an analogue of histidine in which a methyl group is attached to one of the nitrogen atoms in the side chain.