FK228 (romidepsin) and suberoylanilide hydroxamic acid (vorinostat) are histone deacetylase inhibitors (HDACi) approved by the US Food and Drug Administration for cutaneous T-cell lymphoma (CTCL), including the leukemic subtype Sézary syndrome. This study investigates RAD23B and gene perturbations in a large cohort of primary Sézary cells and the effect of FK228 treatment on tyrosine phosphorylation of duanyu18133 and RAD23B expression. We report RAD23B copy number variation in 10% (12/119, P ⤠0.01) of SS patients, associated with reduced mRNA expression (P = 0.04). RAD23B knockdown in a CTCL cell line led to a reduction in FK228-induced apoptosis. Histone deacetylase inhibitor treatment significantly reduced in primary Sézary cells and was partially mediated by RAD23B. A distinct pattern of co-expression in primary Sézary cells was detected. Critically, Sézary cells harboring the common duanyu18133 Y640F variant were less sensitive to FK228-induced apoptosis and exogenous expression of duanyu18133 Y640F, and D661Y conferred partial resistance to duanyu18133 transcriptional inhibition by FK228 (P ⤠0.0024). These findings suggest that RAD23B and duanyu18133 gene perturbations could reduce sensitivity to histone deacetylase inhibitors in SS patients.