Although MBCs with spindle cell metaplasia more frequently displayed PIK3CA mutations than MBCs with other types of metaplasia, PIK3CA mutations were also found in MBCs with squamous differentiation.9 These results suggest that the differences in gene expression between MBCs with or without spindle cell metaplasia may stem from other types of somatic genetic alterations or epigenetic modifications or distinct cells of origin. Here, PIK3CA is linked to maternal uniparental disomy of chromosome 20.