Primary aldosteronism is the most common secondary cause of hypertension with a prevalence of about 10%. About half of PA cases are caused by aldosterone-producing adenomas (APA). Two APA subtypes, ZG-like and ZF-like APAs, have been described, according to the histological resemblance to normal zona glomerulosa (ZG) and zona fasciculata (ZF), underlying somatic mutations (KCNJ5 commonly found in ZF-like, CACN1AD, ATP1A1, ATP2B3, CTNNB1 in ZG-like APAs), and transcriptome profile. It is unknown if the process of tumorigenesis differs between ZG- and ZF-like APAs. In order to define ZG specific genes, we have compared the transcriptome of APAs and their adjacent adrenal glands by microarray assay. RNA was isolated by laser capture microdissection (LCM) from adjacent ZG, ZF and APAs from 14 patients with Conn’s and 7 patients with phaeocromocytoma. Two top hit genes from the comparison of ZG vs ZF were functionally studied, ANO4 and NEFM. NEFM, encoding neurofilament medium, was the fourth most up-regulated gene in ZG vs ZF, showing 14.8-fold-fold higher expression levels (p=9.16-12) in ZG than ZF. NEFM was also one of the most down-regulated genes in ZF-like vs ZG-like APAs. Immunohistochemistry (IHC) confirmed selective high expression of NEFM in ZG and ZG-like APAs. Silencing NEFM in H295R cells increased aldosterone secretion and cell proliferation. In addition, it increased stimulation and inhibition, respectively, of aldosterone secretion from H295R cells by the dopamine receptor D1R agonist fenoldopam and antagonist SCH23390. IHC showed predominantly intracellular staining for D1R in NEFM-rich ZG-like APAs, but membranous staining in NEFM-poor ZF-like APAs. Aldosterone secretion in response to fenoldopam in primary cells from ZG-like APAs was lower than in cells from ZF-like APAs. NEFM expression levels directly correlate with KCNJ5 phenotype: KCNJ5 mutations down-regulate NEFM mRNA and protein levels in H295R cells and in primary cells from ZG-like APAs. ANO4,encoding a Ca2+-activated chloride channel family member, was the third most upregulated gene, showing 19.9-fold higher expression levels (p=6.6x10-24) in ZG than ZF. IHC confirmed ZG selectivity of ANO4 protein in the adrenal cortex. The staining was mainly cytoplasmic. Unlike NEFM, there was no difference in expression of ANO4 between ZG- and ZF-like APAs, the levels being mid-way between those of ZF and ZG. Overexpression of ANO4 in H295R cells caused an increase in CYP11B2 and NR4A2 gene expression levels but basal aldosterone secretion was unchanged. In the presence of calcium agonists, ANO4 reduced aldosterone secretion. ANO4 subcellular localisation was confirmed as cytoplasmic by immunofluorescence microscopy of transfected cells. When exposed to calcium ionophores, ANO4 generated small chloride currents as detected by YFP assay. In summary, the comparison of transcriptome of ZG with paired ZF found unexpected up-regulated genes. Most of the highly up regulated genes in human ZG, including NEFM and ANO4, inhibit either basal or stimulated aldosterone secretion, and this may reflect an adaptive response to high salt intake. No clear-cut correspondence was found between transcriptome of APAs and their resembling zone of adrenal cortex. The down-regulation of NEFM following transfection of mutant KCNJ5 suggests that ZF-like properties may be a consequence of mutation, rather than tissue of origin.