yEnc assumes that binary data mostly can be transmitted through Usenet and email. Therefore, 252 of the 256 possible bytes are passed through unencoded as a single byte, whether that result is a printable ASCII character or not. Only NUL, LF, CR, and = are escaped. LF and CR are escaped because the RFCs that define Internet messages still require that carriage returns and line feeds have special meaning in a mail message. = is the escape character, so it itself is escaped. NUL is also escaped because of problems handling null characters in common code, although as an optimization yEnc adds 42 to every source byte so that, not uncommon, long stretches of zero bytes do not require a lot of escaping.