OPRK1 and heroin dependence: Directly, our data prove changes in the expression of opioidergic genes which, according to GeneCards and MalaCards databases (https://www.malacards.org/card/opiate_dependence (accessed on 3 July 2021)), code for proteins implicated in opioid dependence and addiction: that is, OPRM1 (opioid dependence); OPRM1, OPRK1, OPRD1, and DRD2 (opioid addiction); PENK, PDYN, OPRM1, OPRK1, and OPRD1 (morphine dependence); and PDYN, OPRM1, OPRD1, and DRD2 (heroin dependence).