IMiDs induce a maturational block of granulocytic progenitors. Purified CD34⁺ cells were cultured with thalidomide, lenalidomide, or pomalidomide and analyzed for (A) CD33, (B) CDIIa, (C) CD34, and (D) glycophorin A expression at day 0, 3, 6, and 9 by fluorescence-activated cell sorter analysis. Results are shown as mean ± SD. (E) Table represents the ratio of the myeloid precursors/erythroid precursors in 6 patient bone marrow aspirates, before treatment and at neutrophil nadir induced by lenalidomide. (F) Representative bone marrow aspirates before treatment and at neutrophil nadir from a patient receiving lenalidomide. Arrows indicate immature myeloid cells. Note: during grade 4 neutropenia, the aspirate remains cellular, with a predominant population of neutrophilic precursors. Slides were evaluated using an Olympus BX45 microscope equipped with a 100×/1.35 numeric aperture (Olympus; original magnification ×1000).