Two-dimensional western blot analysis of vitamin D-binding protein isoforms in responders and nonresponders. For analysis of vitamin D-binding protein (VDBP) expression, two-dimensional western blot (2D-WB) analysis was performed. Different isoforms of VDBP were detected at V1 in responders (R) and nonresponders (NR). 2D-WB revealed an additional VDBP isoform in R compared with NR, suggesting a posttranslational modification such as changes in glycosylation pattern. 2D-DIGE, two-dimensional differential gel electrophoresis.