Macro neede to open text files from folder and remove all the lines except specific lines

Hi to all VBA Programmers,

I have a folder, it contains some 15 text files. Each files contains some headings like -chromosomes from 1 to 12 and some lines under each chromosome. Now i want keep one heading (for example Chromosome1) with its lines and remove rest of the headings. for example i want to keep this -Chromosome 1 and its lines (BTX) until next _Chromosome start like this i want do it for remaining -Chromosomes) I am doing this manually but it is taking lot of time to complete one single time like this i have to do 12 times.
Can any one help me with macro code? if yes your efforts would be highly appreciated. I am attching one of the sample file for better understanding of what i am saying.
Thanks in advance,

Regards,
Genetist