HYL1 is a nuclear protein involved in the processing of miRNAs but its exact function remains unknown. Arabidopsis thaliana hyl1 mutants exhibit hypersensitivity to ABA. We decided to answer the question whether ABA affects the HYL1 protein localization within the cell and show that it does not. We also studied the expression of HYL1 in different tissues and organs. In this paper we show for the first time the expression profile of the HYL1 protein using anti-HYL1 antibodies. The protein is present in seedlings and mature plants in all organs studied, with the highest amount in inflorescences. A. thaliana HYL1 protein has several repetitions of a 28-amino-acid sequence at the C-terminus that confer protein instability. Our bioinformatic analysis of HYL1 homologs in different Brassica species shows that this repetition is typical only for Arabidopsis. This may suggest a relatively late evolutionary acquisition of the C-terminal domain.