MSH6 and neoplasm: Loss of ARID1A protein expression was confirmed by IHC, suggesting that the two ARID1A mutations may be in trans, or that mutation of one allele is sufficient for loss at the protein level as observed in other tumor types.13 The patient had detectable levels of ARID1A, CHEK1, FANCM, RAD50, PTEN, MLH1, and MSH6 mutations in circulating free DNA at baseline.