PAX5 and neoplasm: <h4>Introduction</h4>Histiocytic sarcoma (HS) is a rare neoplasm derived from non-Langerhans histiocytic cells, exceptionally arising from B-ALL.<h4>Methods</h4>We present the case of a child with high-risk B-ALL with PAX5 P80R mutation.<h4>Results</h4>Despite initial remission, a chemoresistant paravertebral mass was identified as HS.