Genome-wide sequence analysis in the invertebrate chordate, Ciona intestinalis, has provided a comprehensive picture of immune-related genes in an organism that occupies a key phylogenetic position in vertebrate evolution. The pivotal genes for adaptive immunity, such as the major histocompatibility complex (MHC) class I and II genes, T-cell receptors, or dimeric immunoglobulin molecules, have not been identified in the Ciona genome. Many genes involved in innate immunity have been identified, including complement components, Toll-like receptors, and the genes involved in intracellular signal transduction of immune responses, and show both expansion and unexpected diversity in comparison with the vertebrates. In addition, a number of genes were identified which predicted integral membrane proteins with extracellular C-type lectin or immunoglobulin domains and intracellular immunoreceptor tyrosine-based inhibitory motifs (ITIMs) and immunoreceptor tyrosine-based activation motifs (ITAMs) (plus their associated signal transduction molecules), suggesting that activating and inhibitory receptors have an MHC-independent function and an early evolutionary origin. A crucial component of vertebrate adaptive immunity is somatic diversification, and the recombination activating genes (RAG) and activation-induced cytidine deaminase (AID) genes responsible for the Generation of diversity are not present in Ciona. However, there are key V regions, the essential feature of an immunoglobulin superfamily VC1-like core, and possible proto-MHC regions scattered throughout the genome waiting for Godot.

The self-and mutual-avoiding effects of two star polymers in a good solvent are studied by means of a simple Monte Carlo sampling technique on a cubic lattice, using an enrichment algorithm. The total number of configurations is determined as a function of the distance between the two star polymers. The radius of gyration of a single star polymer, the second virial coefficient, the effective interstar potential, and the penetration function are evaluated for 3-to 6-arm star polymers. Considerable deviation from the result of the previous naive first-order -expansion is observed in the penetration function. We also discuss the value of the penetration function in the infinite-arm limit, according to the cone picture.

Star chains composed of arms with uniform length were generated on a simple cubic lattice by the Monte Carlo method using the enrichment algorithm, which enabled us to obtain chains with total segment numbers (N) up to 1200. The number of arms (f) ranged from 2 to 24. The radius of gyration (〈S 2 〉 1/2 ), the dimensionless intrinsic viscosity (E), and the hydrodynamic radius (RH) were evaluated as functions of f and N by averaging over more than 4000 samples. The calculations of E and RH were made according to Zimm's method. It was found that the radius expansion factor (RS) for star chains with long arms obeys the relation RS 2 ) 1.53z 2/5 , which is known for long flexible linear chains, where z is the excluded volume parameter. The ratios of 〈S 2 〉, E, and RH of star chains to those for linear chains were evaluated as functions of f. These ratios obtained and the Flory factors for the intrinsic viscosity and RH extrapolated to N -1/2 f 0 reproduced experimental data well.