Potassium channel subunit that does not form functional channels by itself. Can form functional heterotetrameric channels with KCNB1 and KCNB2; modulates the delayed rectifier voltage-gated potassium channel activation and deactivation rates of KCNB1 and KCNB2 (PubMed:10484328). Heterotetramer with KCNB1 (PubMed:10484328). Heterotetramer with KCNB2 (By similarity). Does not form homomultimers (PubMed:10484328). May not reach the plasma membrane but remain in an intracellular compartment in the absence of KCNB1 or KCNB2 (PubMed:10484328). Detected in all tissues tested with the exception of skeletal muscle. Highly expressed in adult and fetal brain, fetal kidney and lung, and adult prostate and testis (PubMed:10484328). The transmembrane segment S4 functions as voltage-sensor and is characterized by a series of positively charged amino acids at every third position. Channel opening and closing is effected by a conformation change that affects the position and orientation of the voltage-sensor paddle formed by S3 and S4 within the membrane. A transmembrane electric field that is positive inside would push the positively charged S4 segment outwards, thereby opening the pore, while a field that is negative inside would pull the S4 segment inwards and close the pore. Changes in the position and orientation of S4 are then transmitted to the activation gate formed by the inner helix bundle via the S4-S5 linker region. Belongs to the potassium channel family. S (TC 1.A.1.2) subfamily. Kv9.1/KCNS1 sub-subfamily.