Because ALDH1 positivity, as determined by Aldefluor analysis, reflects the combined output of different aldehyde dehydrogenase activities – namely ALDH1A1, ALDH1B1 and ALDH1A3 et al. – we here dissected the ALDH1+ phenotype and demonstrated that ALDH1A1 is the predominant isoform in myeloma. Here, ALDH1A1 is linked to plasma cell myeloma.