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We present here a case study of a neonate diagnosed with T21 mosaicism and TAM who did not exhibit the typical phenotypic features of down syndrome (DS), but displayed hematologic abnormalities, in addition to hepatosplenomegaly. Initial genetic testing suggested acute myeloid leukemia (AML) but subsequent evaluations were indicative of T21 mosaicism confined to the myeloid cell line, with negative results from lymphocytes cultured from a skin biopsy. A pathogenic <jats:italic>GATA1<\/jats:italic> variant was found in the bone marrow in addition to three copies of <jats:italic>RUNX1<\/jats:italic>, associated with aberrant hematopoiesis in TAM. The infant responded to a brief course of chemotherapy and demonstrated normal growth and development at four years of age. In addition to this case, we identified 25 cases from the literature of mosaic T21 restricted to the myeloid cell line supporting normal development following treatment for TAM. As this case and the literature review demonstrate, T21 mosaicism apparently isolated to the bone marrow is unlikely to be associated with systemic or neurodevelopmental manifestations of DS.<\/jats:p>","DOI":"10.1002\/ajmg.a.63979","type":"journal-article","created":{"date-parts":[[2025,2,26]],"date-time":"2025-02-26T06:47:14Z","timestamp":1740552434000},"update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":1,"title":["Presentation and Longer\u2010Term Outcomes in Mosaic Trisomy 21 Causing Isolated Transient Abnormal Myelopoiesis"],"prefix":"10.1002","volume":"197","author":[{"ORCID":"https:\/\/orcid.org\/0009-0009-7994-5018","authenticated-orcid":false,"given":"Anna\u2010Th\u00e9r\u00e8se","family":"Mehra","sequence":"first","affiliation":[{"name":"Division of Newborn Medicine, Department of Pediatrics Boston Children's Hospital and Harvard Medical School  Boston Massachusetts USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-8162-5031","authenticated-orcid":false,"given":"Monica H.","family":"Wojcik","sequence":"additional","affiliation":[{"name":"Division of Newborn Medicine, Department of Pediatrics Boston Children's Hospital and Harvard Medical School  Boston Massachusetts USA"},{"name":"Division of Genetics and Genomics, Department of Pediatrics Boston Children's Hospital and Harvard Medical School  Boston Massachusetts USA"},{"name":"The Manton Center for Orphan Disease Research, Department of Pediatrics Boston Children's Hospital and Harvard Medical School  Boston Massachusetts USA"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"311","published-online":{"date-parts":[[2025,2,24]]},"reference":[{"key":"e_1_2_10_2_1","doi-asserted-by":"publisher","DOI":"10.1097\/MPH.0b013e31818a953e"},{"key":"e_1_2_10_3_1","doi-asserted-by":"publisher","DOI":"10.1182\/bloodadvances.2021006865"},{"key":"e_1_2_10_4_1","doi-asserted-by":"publisher","DOI":"10.1101\/mcs.a006126"},{"key":"e_1_2_10_5_1","first-page":"596","volume-title":"Treatment of Infant Leukemias: Challenge and Promise","author":"Brown P.","year":"2013"},{"key":"e_1_2_10_6_1","doi-asserted-by":"publisher","DOI":"10.1016\/j.jpeds.2005.12.031"},{"key":"e_1_2_10_7_1","doi-asserted-by":"publisher","DOI":"10.1182\/blood\u20102011\u201004\u2010350017"},{"issue":"1","key":"e_1_2_10_8_1","first-page":"79","article-title":"Transient Myeloproliferative Syndrome in a Phenotypically Normal Infant","volume":"7","author":"Hanna M.","year":"1985","journal-title":"American Journal of Pediatric Hematology\/Oncology"},{"key":"e_1_2_10_9_1","doi-asserted-by":"publisher","DOI":"10.1002\/ajmg.c.32063"},{"key":"e_1_2_10_10_1","doi-asserted-by":"publisher","DOI":"10.1016\/j.ncl.2016.06.009"},{"key":"e_1_2_10_11_1","doi-asserted-by":"publisher","DOI":"10.1016\/j.dsi.2011.01.005"},{"key":"e_1_2_10_12_1","doi-asserted-by":"publisher","DOI":"10.1200\/JCO.2010.32.3634"},{"key":"e_1_2_10_13_1","doi-asserted-by":"publisher","DOI":"10.5114\/ada.2022.115895"},{"key":"e_1_2_10_14_1","doi-asserted-by":"publisher","DOI":"10.1046\/j.1365\u20102141.1998.00996.x"},{"key":"e_1_2_10_15_1","doi-asserted-by":"publisher","DOI":"10.7759\/cureus.49644"},{"key":"e_1_2_10_16_1","doi-asserted-by":"publisher","DOI":"10.1038\/s41375\u2010020\u20100854\u20105"},{"key":"e_1_2_10_17_1","doi-asserted-by":"publisher","DOI":"10.1155\/2015\/809406"},{"key":"e_1_2_10_18_1","first-page":"123","volume-title":"Chapter\u00a07. 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