On an SDS-PAGE gel a ladder shows 100 kDa protein migrating 2.0 cm from the well and 25 kDa protein migrating 8.0 cm. An unknown protein migrates 5.0 cm. Using log10(MW) linear interpolation between the two ladder points, estimate the molecular weight of the unknown protein. Show calculations.

• A. Approximately 25 kDa, since migration past the midpoint always corresponds to the lower marker and the unknown migrated farther than the average distance so it should be equal to the smallest marker in the range.
• B. Approximately 75 kDa, because the unknown is closer to the 100 kDa band and therefore must be three quarters of the distance between markers and correspondingly higher in mass.
• C. Approximately 100 kDa because the unknown mass is estimated from the nearest upper marker when interpolation is ambiguous and the safer clinical assumption is to report the higher mass.
• D. Approximately 50 kDa (calculated by log-linear interpolation of migration distances).

Answer: (D)