  Preliminary results of a systematic study on the simultaneous
optical-to-X-rays variability in blazars are presented. Data from Swift
observations of four bright gamma-ray blazars (3C 279, ON+231, S5 0716+71, PKS
2155-304) have been analyzed, compared, and discussed. Specifically, 3C 279
shows a variable flattening in the low energy part of the X-ray spectrum that
appears to be confined in a specific X-ray vs optical/UV fluxes region. Some
implications are shortly analyzed.
