Description: Microglandular adenosis (MGA) is a rare proliferative lesion of the breast composed of small glands lacking myoepithelial cells and lined by S100-positive, oestrogen receptor (ER)-negative, progesterone receptor (PR)-negative and HER2-negative epithelial cells. There is evidence to suggest that MGA may constitute a non-obligate precursor of triple-negative breast cancer (TNBC). We sought to define the genomic landscape of pure MGA and atypical MGA (AMGA) {EdQ; Please ensure your meaning has been retained} and of associated TNBCs, and to determine whether synchronous MGA, AMGA and TNBCs could be clonally related. Two pure MGAs and eight cases of MGA and/or AMGA associated with in situ or invasive TNBC were collected, microdissected and subjected to massively parallel sequencing that targeted all coding regions of 236 genes recurrently mutated in breast cancer or related to DNA repair. Pure MGAs lacked clonal non-synonymous somatic mutations and displayed limited copy number alterations (CNAs); conversely, all MGAs (n=7) and AMGAs (n=3) associated with TNBC harboured at least one somatic non-synonymous mutation (range 3-14 and 1-10, respectively). In all cases where TNBCs were analysed, identical TP53 mutations and similar patterns of gene CNAs were found in the MGA and/or AMGA and in the associated TNBC. In the MGA/AMGA associated with TNBC lacking TP53 mutations, somatic mutations affecting PI3K pathway-related genes (e.g. PTEN , PIK3CA and INPP4B ) and tyrosine kinase receptor signalling-related genes (e.g. ERBB3 and FGFR2 ) were identified. At diagnosis, MGAs associated with TNBC were found to display subclonal populations and clonal shifts in the progression from MGA to AMGA and/or to TNBC were observed. Our results demonstrate the heterogeneity of MGAs, and that MGAs associated with TNBC, but not necessarily pure MGAs, are genetically advanced, clonal and neoplastic lesions, harbouring recurrent mutations in TP53 and/or other cancer genes, supporting the notion that a subset of MGAs and AMGAs may constitute non-obligate precursors of TNBCs.