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The absence of a sex chromosome in conjunction with the presence of a marker chromosome generally implicates a sex chromosome origin for such marker chromosomes. These types of findings are frequently associated with Ullrich–Turner syndrome. We report a patient that presented with an atypical Ullrich–Turner phenotype and a cytogenetic mosaicism of 46,X,mar/46,XX. The marker chromosome was derived from chromosome 20, not from the X or Y chromosome. The patient's clinical features are described and discussed relative to the cytogenetic findings. This case further demonstrates the necessity of marker chromosome identification for accurate phenotype–karyotype correlation. [ABSTRACT FROM AUTHOR]