Among the chosen MLPs candidates, the importance of several in NRP biosynthesis has been demonstrated in previous studies. For instance, CdaX is encoded by a gene from the known calcium-dependent peptide antibiotic (CDA) BGC in _S. coelicolor_ (Table 3) and can functionally replace CchK, which is encoded in the gene cluster responsible for producing the siderophore coelichelin in the same organism. The deletion of either _cdaX_ or _cchK_ reduces but does not abolish the production of the respective NRP products, while the disruption of both genes completely eliminates the production of both metabolites (Lautru et al., 2007). Additionally, CdaX has been shown to stimulate the activities of L-tyrosine-adenylating enzymes from different NRP biosynthetic pathways (Boll et al., 2011). In contrast, results from our previous study suggested that CdaX is unable to functionally replace TxtH in the thaxtomin biosynthetic pathway (Li et al., 2019a). CloY from the clorobiocin BGC of _S. roseochromogenes_ (Table 3; Pojer et al., 2002) is essential for production of the aminocoumarin antibiotic (Wolpert et al., 2007), as it is required for the solubility and adenylation activity of its corresponding NRPS partner, CloH (Boll et al., 2011). The ComB-encoding gene is situated within a glycopeptide-like complestatin NRP BGC from _S. lavendulae_ (Chiu et al., 2001) and was recently shown to stimulate the production of several NRPs in the mold _Penicillium chrysogenum_, which does not harbor any MLP-encoding genes in its genome (Zwahlen et al., 2019). YbdZ has been extensively investigated in recent studies and is required for the biosynthesis of the ENT siderophore in _E. coli_ (Schomer and Thomas, 2017; Schomer et al., 2018). The deletion of _ybdZ_ abolishes ENT production even though its NRPS partner (EntF) is not dependent on the presence of YbdZ for soluble protein production, and biochemical analyses have shown that the solubility and catalytic activity of EntF is significantly enhanced by YbdZ (Felnagle et al., 2010). PA2412 is the MLP associated with the biosynthesis of the siderophore pyoverdine in _P. aeruginosa_, and strains without PA2412 cannot produce pyoverdine or grow under iron-restricted conditions (Drake et al., 2007). Furthermore, PA2412 has the ability to promote ENT biosynthesis in _E. coli_ in the absence of YbdZ (Schomer and Thomas, 2017). Intriguingly, the orphan MLP MXAN_3118 from _M. xanthus_ is not encoded within any NRP BGC, but it is able to interact with seven different NRPSs that are encoded elsewhere in the genome of this organism (Esquilín-Lebrón et al., 2018). In addition, MXAN_3118 can functionally replace YbdZ in multiple assays conducted in _E. coli_ (Schomer and Thomas, 2017) and is therefore thought to be a promising “universal” MLP for promoting heterologous expression of NRPSs in bacterial and fungal strains in order to improve metabolite production.