I an currently working on a project that will involve the Horvath epigenetic clock as a quantitative measure of ageing. The clock is based on assessing the methylation of 353 CpG sites. I'm new to this area, but it appears that the standard approach is to use the methylation array chips from illumina. I'v found that the current Illumina chip (850k) is fairly expensive and lacks coverage of 17 CpG sites required for the clock.

Does anyone know of an alternative for assessing these specific 353 CpG sites?

Or

A method to help narrow down which sample are worth analyzing with the chip?

Thanks