Cell states in hematopoiesis are controlled by master regulators and by complex 
circuits of a growing family of RNA species impacting cell phenotype maintenance 
and plasticity. Circular RNAs (circRNAs) are rapidly gaining the status of 
particularly stable transcriptome members with distinctive qualities. RNA-seq 
identified thousands of circRNAs with developmental stage- and tissue-specific 
expression corroborating earlier suggestions that circular isoforms are a 
natural feature of the cell expression program. CircRNAs are abundantly 
expressed also in the hematopoietic compartment. There are a number of studies 
on circRNAs in blood cells, a specific overview is however lacking. In this 
review we first present current insight in circRNA biogenesis discussing the 
relevance for hematopoiesis of the highly interleaved processes of splicing and 
circRNA biogenesis. Regarding molecular functions circRNAs modulate host gene 
expression, but also compete for binding of microRNAs, RNA-binding proteins or 
translation initiation and participate in regulatory circuits. We examine 
circRNA expression in the hematopoietic compartment and in hematologic 
malignancies and review the recent breakthrough study that identified pathogenic 
circRNAs derived from leukemia fusion genes. CircRNA high and regulated 
expression in blood cell types indicate that further studies are warranted to 
inform the position of these regulators in normal and malignant hematopoiesis.