ARID1A and neoplasm: The presence of concomitant mutations in the ARID1A gene in the same sample may be explained with two different biological conditions: (i) the same clonal neoplastic cells that harbor one ARID1A mutation on one allele and the other alteration on the other allele; or (ii) tumor heterogeneity, i.e., different clones of neoplastic cells that harbor different ARID1A mutations.