In our study with 271 cases with confirmed CBFB rearrangement identified from over 1600 AML cases by CBFB FISH and CBFB-MYH11 RT-PCR, over 5% of them initially presented as challenging results, including discrepancy between FISH and RT-PCR tests and/or atypical FISH findings, which are mostly caused by additional chromosome 16 aberrations (AC16As). The gene discussed is CBFB; the disease is acute myeloid leukemia.