The recent literature attests to the increasing number of enzymes for which isoenzymes or subunit structure or both are known. Several symposia and reviews have been published within the past few years.46–50Some enzyme activities for which isoenzymes have been described are listed in table 4. Only those found in mammals are included. They are ranked in rough order of apparent usefulness in clinical diagnosis, with the enzymes displaying tissue localization and catalytic differences listed first. Also, the number of molecular forms and the presence or absence of known subunit structure is indicated.Clinical studies have been made using several of the isoenzymes listed in table 4, and references denoting these studies are marked with an asterisk. Data presented in the table indicate that valuable clinical information might derive from study of other isoenzymes, notably phosphofructokinase, for which tissue differences and adaptable catalytic differences exist. The guidelines set forth in this review can be used for investigating still other isoenzymes, such as those studied in lower forms.79–85In brief, the study of isoenzymes, like the study of enzymes and other organ-specific chemicals, presents an opportunity for great specificity and accuracy in localizing and following disease processes.