Gay and Stephens (29), and Del Grosso et al. (23) and were shown not only to be quite different but also to behave quite differently. The two genes have disseminated markedly differ- ently and are being recognized in an ever growing number of microbial species. At present, both the mef(A) and the mef(E) genes have unambiguously been identified in five streptococcal species, whereas mef(E) has been identified in five more strep- tococcal species and in nine additional nonstreptococcal spe- cies (Table 1). Furthermore, Amezaga et al. (5) reported that the MICs for mef(A)-containing S. pneumoniae isolates were significantly higher than those for mef(E)-containing isolates. This indicates that, despite the high degree of homology be- tween the two genes, in the context of the genome in which they are embedded, the differences between them are sufficient to impose different susceptibility characteristics on the strains carrying the genes. The existence of these differences between the two genes has prompted others to suggest that the differ- ence between the two genes be maintained (23, 55) and may have been one reason(s) why others also continued using the names mef(A) and mef(E) after publication of the nomencla- ture minireview (5, 12, 13, 15, 19, 23, 35, 54, 55, 66). As a result of this, there is no widespread consensus about the nomenclature for the mef genes in the present literature. For readers unaware of this, this may give rise to conflicting interpretations of the available literature and resources on the subject. In this minireview, we outline how the use of different mef gene nomenclatures has created considerable confusion in the field. We also review the current resources on the subject: we performed a search for mef gene sequences in public DNA databases and looked at methods for the detection of mef genes in clinical isolates and tools that can be used to discrim- inate between mef(A) and mef(E). This information was then used to review the data in the literature with respect to re- ported genes versus the actual genes that were studied, given the information in the methods sections.