Slowing down of the maturation process of human rotavirus particles on ice allowed the clear demonstration of two different assembly pathways through the endoplasmic reticulum (ER) membrane. One was the 'enveloped' and single-shelled (ss) particle assembly pathway, in which a transient envelope is acquired through the budding of subviral particles from the cytoplasm to the ER lumen, and later these 'enveloped' particles are released as ss particles in the ER lumen. The other was a double-shelled particle assembly pathway by which subviral particles acquire the outer capsid proteins during their transport across the ER membrane.