LAUSR

chordomas with epithelioid and focal rhabdoid morphology. However, it typically retains brachyury expression and occurs in different demographic (median age of 11 years; female-to-male ratio of 2). Moreover, unlike AT/RT, the mechanism of SMARCB1/INI1 loss in chor-domas involves homozygous deletion of SMARCB1 gene. Molecular and cytogenetic analysis are critical for accurate diagnosis. In AT/RT, genetic alterations often include a mutation in one allele of SMARCB1 , followed by a second allele loss due to monosomy 22, deletion of 22q11.2, or an acquired cnLOH [1]. Our case exhibits a cytosine deletion in the exon 9 of SMARCB1 (c.1148del) with concurrent cnLOH of chromosome 22. Interest-ingly, instead of introducing a premature stop codon and truncated protein, this deletion leads to a frameshift mutation in the last three codons, resulting in the addition of 99 extra amino acids before termination. Mecha-nistically, the elongated protein is predicted to be inactive, but how this results in loss of SMARCB1/INI1 expression, as shown by negative immunostaining, requires further investigation. Finally, methylation profiling of pediatric ATRT has established three distinct subtypes based on the overexpression of melanosomal genes (TYR), sonic hedgehog pathway (SHH), and MYC and HOX cluster. Consistent with recent findings that seven adult sellar AT/RTs are classified as ATRT-MYC subtype [2], our case was classified as an ATRT-MYC subtypes by the NIH classifier.