This problem is most likely caused by a mismatch in ionic strength (salt concentration) between the molecular weight marker and samples. A high salt concentration in the sample will cause it to spread sideways if it is adjacent to a lane in which the salt concentration of the sample is much lower. The solution to this problem, in this case, would be to add salt to the marker to be similar to the salt concentration in the other samples.

I agree with Adam B Shapiro that it most likely is a salt issue. The other solution would be to leave a spacer lane between the ladder and the samples if you can't figure out what to add to the markers.

Hello,

If the other samples in the SDS-PAGE gel are well-resolved and spread out, but the protein ladder appears as dots, it could be due to uneven loading of the ladder onto the gel. The ladder may not have been evenly dispersed and loaded onto the gel, resulting in localized areas of higher concentration that appear as dots. Alternatively, it could also be due to the ladder itself. The ladder may have been improperly stored or handled, resulting in aggregation or impurities that cause dot-like artifacts on the gel. To minimize the problem, it is important to ensure even loading of the ladder, proper storage and handling of the ladder to prevent aggregation, and careful preparation and handling of the gel to ensure consistent polymerization and drying. Additionally, it may be helpful to use a fresh or different protein ladder to confirm that the dots are not due to ladder-related issues.