Larger Version | Mice with a defect in the gene ALX4 grow extra toes on their back feet (top left). Back feet with extra digits are a hallmark of the Great Pyrenees dog breed (top right). Scientists have associated the extra toe with a deletion of repeated amino acids in the breed’s ALX4 protein. In tests of 89 dog breeds, researchers found that the ALX4 gene was shorter only in Great Pyrenees dogs with an extra toe (gels, middle, compare ALX4 from 89 breeds; arrow shows Pyrenees’ short version). The short gene results in the loss of a 17 amino-acid repeat (highlighted in the amino acid sequence from other dog breeds) from the ALX4 protein in Great Pyrenees (site of deletion marked on sequence, bottom). Credit: Fondon/University of Texas at Arlington