This protein family consists of HisA, phosphoribosylformimino-5-aminoimidazole carboxamide ribotide isomerase <db_xref db="EC" dbkey=" 5.3.1.16"/>, the enzyme catalyzing the fourth step in histidine biosynthesis. It is closely related to the enzyme HisF for the sixth step. Examples of this enzyme in Actinobacteria have been found to be bifunctional, also possessing phosphoribosylanthranilate isomerase activity [<cite idref="PUB00013823"/>].